Which is the "safer"/"more Biblical" view, and why?
My instinct is that it might be safer to understand the Dichotomous view primarily based on my own experience in transitioning from a Trichotomous view early in my Christian life. Having said that, someone like A.W Pink probably has a healthy version of the a Trichotomous view and I would not call it heretical.
My introduction to the Trichotomous view was very early in my Christian life by reading Watchman Nee and just a few years later I tossed out his books as I had then encountered Dr. Martin Lloyd-Jones and saw a clear difference between Biblical exposition and a kind of mysticism that Nee promoted. The reason why the Trichotomous view (in this case) led to a sort of mysticism is that the ‘spirit’ as ‘opposed’ to the ‘soul’ was focused on as the true Godly part, and one was encouraged to sort of ignore the mind, emotions and will and try to reach out for a more ‘spiritual’ state ---- in communion with God.
As I started to read older Bible commentaries and works from reformed theologians from the 16-18th centuries, I noticed a switch to a Dichotomous view that seemed more aligned with a spiritual sense of God through understanding of his Word. To make a long story short I sort of adopted the Dichotomous view without really thinking about it.
Now many years later I will just summarize the perspective that I have found in various older Bible commentaries, in support of the Dichotomous view.
First let me quote a book that others have noted that provides a good summary of both views:
The dichotomous theory.
Strong states the theory thus:
The immaterial part of man, viewed as an individual and conscious life, capable of possessing and animating a physical organism, is called psuche; viewed as a rational and moral agent, susceptible of divine influence and indwelling, this same immaterial part is called. The pneumu, then, is man’s nature looking and capable of receiving and manifesting the Pneumu the psuche is man’s nature looking earthward, and touching the world of sense. The pneumu is man’s higher part, as related to spiritual realities or as capable of such relation. Man’s being is therefore not trichotomous but dichotomous, and his immaterial part, while possessing duality of powers, has unity of substance.
The trichotomous theory
This theory holds that man consists of three distinct elements: body, soul, and spirit. The body is the material part of our constitution, the soul is the principle of animal life, and the spirit is the principle of our rational life. Some add to this last statement “and immortal life.” This can, however, not be made an essential part of the theory. Those who take this extreme view hold that at death the body returns to the earth, the soul ceases to exist, and the spirit alone remains to be reunited with the body at the resurrection. (Lectures In Systematic Theology, Henry Clarence Thiessen, P160)
Second let me explain why a good understanding of the dichotomous theory does have some similarities with the trichotomous theory, but better explains that commonality.
The basic situation when looking at many places in scripture where the soul or spirit are spoken of, one quickly finds that the meanings overlap but also convey separate senses. Therefore in general the spirit means something different than the soul even though they are indicating the same vital non physical lfe of man.
From a basic standpoint of occurenes in the Bible in the Old Testament נָ֫פֶשׁ (soul) is used almost always as the English ‘soul’ but commonly just ‘life’ or functions of the mind, will or heart that are more often related to ‘desire’. In the New Testament ψυχῆς (soul) is alsmot always ‘soul’ or ‘life’. Looking at the word ‘spirit’ we find similar but different meanings. ר֫וּחַ (spirit) in the Old Testament is almost always ‘Spirit’, ‘spirit’, or ‘wind’. In the New Testament (πνεύματος) it is almost always just ‘Spirit’ or ‘spirit’.
I think this establishes Strong’s definition quite well for the soul is therefore the essential life of man ‘looking earthward’ and the ‘spirit’ that same principle of life breathed (like wind) into man from God, that can look towards and experience God. One must accept this different cast used by spirit or soul upon the principle of life in man, because later the idea of carnal and spiritual are assigned to these different aspects. This is where the commonality of both views occur.
For example, in 1 Corinthians 2:14-15 we have a phrase a ‘natural man’ does not accept the things of the Spirit of God. Here the natural man is ψυχικὸς psuchikos (natural, of the soul or mind) is used which has a strong relationship to the word soul (ψυχῆς - psychēs). In verse fifteen in contrast to the carnal man, we find the phrase 'he that is spiritual'. Spiritual πνευματικὸς (pneumatikos) has a strong relationship to the word 'spirit' πνεύματος (pneumatos). So sometimes 'soul' indicates an identification with the unregenerate parts of man' soul (or essential life) being considered 'carnal' and the regenerate parts of man’s soul (called spirit) being spiritual. This division is what commentators often notice. For example, in Hebrews where the word of God is said to divide spirit and soul, the carnal and spiritual parts of a man might be understood.
The other thing of notice, that I have found commentaries often do, when they context does not allow a carnal or spiritual division, is refer to the 'seat of emotions' as the soul and the 'mind' denoted by spirit, such as loving God with 'soul and mind', or 'who knows the thoughts of a man accept the spirit of a man'? (Math 22:37, 1 Cor 2:11)
Conclusion: A dichotomous view is a safer view as it seems more grounded in the Bible and in reformed theology. In an in-depth analysis of the soul and the spirit, one finds different aspects of the 'same single vital immaterial life God breathed into Adam' called soul or spirit. However, the soul and spirit often denote different aspects of that life. In a believer, in the struggle of sin, they often denote the difference between the carnal and spiritual mind and affections, for the soul looking heavenward (spirit) is very actively opposing its former lusts (soul looking earthward). When this distinction is 'over pressed', however, a 'Trichotomous view' is established that is not common with earlier theologians. In its more extreme version, like in Watchman Nee, it starts to promote ‘super spirituality’ and ‘mysticism’ that could become ‘less safe’ as it no longer rests on spirituality by God's word but by 'inner motions' and 'senses' - actually encouraging a carnal view of the spiritual life.