Both Mormons and Jehovah's Witnesses go from door to door, talking to people about their beliefs. Also neither group accepts the notion of Trinity.
What are the main differences between these two denominations?
There are many doctrinal and practical differences between Jehovah's Witnesses and Mormons. I think it will be difficult to arrange a complete and comprehensive list, but below are a collection of differences that immediately come to mind.
Mormons claim to be a restoration of ancient Christianity via divine intervention and angelic visitation. Jehovah's Witnesses claim to be a rediscovery of ancient Christianity through biblical scholarship.
Jehovah's Witnesses hold the use of the correct name of God, Jehovah, to be very important to true worship. Mormons do have beliefs about the correct names of God the Father and Jesus Christ, but do not hold them to the same level of importance as Jehovah's Witnesses do.
Mormons believe that Jesus Christ is Jehovah, that Adam is Michael the Archangel, and that the true name of God the Father is Elohim. Jehovah's Witnesses believe that Jesus Christ is Michael the Archangel, and that the true name of God the Father is Jehovah.
Mormons believe that the Bible contains errors and rely on additional books of scriptures and revelations from modern prophets to interpret it. Jehovah's Witnesses believe that the Bible is completely correct and internally consistent and derive all of their doctrine from their interpretation of it.
Mormons believe in a Spirit World between death and resurrection, Jehovah's witnesses believe that after death the self ceases to exist until resurrection.
Mormons call the Lord's Supper the Sacrament, substitute water for wine, and partake of it weekly on Sundays. Jehovah's witnesses call the Lord's Supper the Lord's Evening Meal and allow only a few worthy to partake of it once a year.
Mormon missionaries are largely young men and women called to serve 18 months to 2 years, other than this, most Mormons do not actively preach door to door. Jehovah's Witnesses believe that it is their responsibility as Christians to witness door to door and hold witnessing as requirement for true membership.
Using the LDS Articles of Faith (in bold-to clarify only) to examine only principle beliefs one can see the similarities and differences.
1 We believe in God, the Eternal Father, and in His Son, Jesus Christ, and in the Holy Ghost.
Mostly aligned except Jehovah Witnesses belief of the Holy Ghost is:
The holy spirit is God’s power in action, his active force. (Micah 3:8; Luke 1:35) God sends out his spirit by projecting his energy to any place to accomplish his will.1
The LDS's belief is that the Holy Ghost is a personage of spirit and a member of the Godhead.
2 We believe that men will be punished for their own sins, and not for Adam’s transgression.
3 We believe that through the Atonement of Christ, all mankind may be saved, by obedience to the laws and ordinances of the Gospel.
Also at first glance may seem similar but it is my understanding that Jehovah Witnesses believe only 144,000 will be resurrected to rule with Jesus in heaven while others will be resurrected and be given a chance to learn God's ways or be destroyed forever.2 -> pt 6 and 9 The LDS believe all will be resurrected and everyone has a chance to be with Jesus in heaven, but those who choose not to follow God's ways will find place in another kingdom of glory.3
4 We believe that the first principles and ordinances of the Gospel are: first, Faith in the Lord Jesus Christ; second, Repentance; third, Baptism by immersion for the remission of sins; fourth, Laying on of hands for the gift of the Holy Ghost.
Jehovah Witnesses have similar beliefs on the first 3 but not on the gift of the Holy Ghost.
5 We believe that a man must be called of God, by prophecy, and by the laying on of hands by those who are in authority, to preach the Gospel and administer in the ordinances thereof.
This is one of the major differences. The LDS believe they have the same Priesthood power and authority that was on the earth when Jesus walked the earth. It seems that Jehovah Witness leaders just need years of study based on this answer.
6 We believe in the same organization that existed in the Primitive Church, namely, apostles, prophets, pastors, teachers, evangelists, and so forth.
7 We believe in the gift of tongues, prophecy, revelation, visions, healing, interpretation of tongues, and so forth.
Both believe in gifts of the spirit but the Jehovah Witnesses believe that the gifts of the spirit stopped with the Apostles.4
8 We believe the Bible to be the word of God as far as it is translated correctly; we also believe the Book of Mormon to be the word of God.
Another distinctive difference is the LDS belief in more scripture: the Book of Mormon, Doctrine and Covenants, and the Pearl of Great Price. The Jehovah Witnesses believe the Bible to be the word of God.2
9 We believe all that God has revealed, all that He does now reveal, and we believe that He will yet reveal many great and important things pertaining to the Kingdom of God.
Similar to Article 5, the LDS are distinct in their belief of modern revelation and to inspired leaders.
10 We believe in the literal gathering of Israel and in the restoration of the Ten Tribes; that Zion (the New Jerusalem) will be built upon the American continent; that Christ will reign personally upon the earth; and, that the earth will be renewed and receive its paradisiacal glory.
Both LDS and Jehovah Witnesses believe in missionary work5 and 6. However the Jehovah Witnesses believe the New Jerusalem to be Heavenly and symbolic. Both also believe that the earth will be transformed into paradise and ruled by Jesus.7
11 We claim the privilege of worshiping Almighty God according to the dictates of our own conscience, and allow all men the same privilege, let them worship how, where, or what they may.
For the most part, Jehovah Witnesses believe similarly.2 and 8
12 We believe in being subject to kings, presidents, rulers, and magistrates, in obeying, honoring, and sustaining the law.
Jehovah Witnesses also believe this.
13 We believe in being honest, true, chaste, benevolent, virtuous, and in doing good to all men; indeed, we may say that we follow the admonition of Paul—We believe all things, we hope all things, we have endured many things, and hope to be able to endure all things. If there is anything virtuous, lovely, or of good report or praiseworthy, we seek after these things.
For the most part, this holds true for Jehovah Witnesses too.9
Over a period of several years studying the beliefs of Jehovah’s Witnesses and Latter Day Saints, I compiled a document comparing similarities and differences. This is a summary of all the differences I found based on information taken from their official web sites. This list simply highlights the main theological differences between the two denominations. I believe it to be accurate, but if there are errors, please let me know.
LDS – Almighty God (Elohim) was once a man JWs – Almighty God (Jehovah) was never created, was never a man, and is Spirit
LDS – The first offspring of Heavenly Father and Heavenly Mother was their spirit son Jehovah, the pre-mortal Jesus JWs – Jehovah has never had a wife but created Jesus (also known as Michael the Archangel)
LDS – Michael the Archangel, the Ancient of Days, helped to create the earth JWs – All things were created by Jehovah through Jesus (also known as Michael the Archangel)
LDS – Michael the Archangel came to earth as Adam and brought Eve, one of his wives JWs – Jehovah, through Jesus, created Adam from the ground and then Eve from Adam’s rib
LDS – Jesus is the Son of God, the Word, I AM, Jehovah, Lord, Messiah and Saviour JWs – Jesus is the Son of Jehovah, Christ, Messiah. “a god,” and was a perfect human
LDS – Jesus has a resurrected physical body JWs – Jesus’ body disappeared and he was resurrected as a spirit
LDS – The Holy Ghost is a spirit person but does not have the qualities of God JWs – The holy spirit is Jehovah’s active force, or energy, but is not a person
LDS – The Bible is accurate only as far as it has been translated properly but the Book of Mormon, given to Joseph Smith, is more accurate JWs – The Bible is Jehovah’s inspired word and no other written word has been issued
LDS – The Aaronic and Melchizedek priesthoods have been restored to LDS JWs – Jehovah chose them as his sole channel of communication, his earthly organization, in 1919
LDS – Gifts of tongues, prophesy, revelation, visions, healings, interpretations exist JWs – The gifts of the spirit ceased after the initial outpouring at Pentecost
LDS – New Jerusalem will be built in the U.S.A. and Christ will reign on earth, a paradise earth JWs – New Jerusalem is heavenly – Christ and 144,000 co-rulers will reign from heaven over a paradise earth during the millennial reign of Christ
LDS – There are three levels of salvation – Telestial, Terrestrial and Celestial Kingdoms JWs – There are two levels of salvation – Heavenly (144,000 spirit anointed co-rulers with Christ) and Earthly (all the survivors of Armageddon plus billions who will be resurrected with physical bodies and given the chance to live forever on a paradise earth)