In the future, I will learn Koine Greek and I will read the New Testament in it’s original language. I will also read the Septuagint. The Dead Sea scrolls are all fragments so They are defined not proper to read the Bible. But which is older? And when were the Dead Sea Scrolls finished being written? God bless you all.
A question of reliability ultimately appeals to a method for determining such, an answer this open is hard to answer except for by the answerers feelings, as is demonstrated in the first answer to this question.
The reliability of the dead sea scrolls is that of fragments and many are isolated fragments, meaning they have no counterpart to know the whole text. And in cases where the text is whole, such as the 'Great Isaiah Scroll', the preservation of the transliteration in to the Masoretic is very close.
However: Presuming that by the word 'reliable' it is meant accurate? The LXX offers more consistent and refined comparison when view beside the totality of the Masoretic text as a standard for comparison. The Dead sea scrolls as fragments then only help in comparing the Greek LXX Septuagint with the 9th-10th century Masoretic Hebrew Text.
Presuming by the word 'reliable' means the doctrinal quality as to attain a more complete doctrine of the testament of scripture prior to the Christian era? The LXX offers a more complete comprehension of the intertestamental period as well as well known working classic language to understand the meaning of certain Hebrew words which the Masoretic text is the possessor of alone (Hebrew which is only found in the bible).