Based on recent manuscript discoveries such as the Dead Sea Scrolls, is the text of the LXX more reliable than the MT?
There are plenty of web sites that will give you comparatives, however, my broad take on the subject is that the LXX is not generally speaking considered more authoritative than any Hebrew text. The translators were not especially careful (though certainly not sloppy.) The amount of textual variants in the Hebrew text are MUCH smaller than in the Greek, for a number of reasons (one being that Hebrew bibles tended to get destroyed, so that before the DSS, the earliest we had was from the 11th century.) However, there is a whole other matter which is that the text of the Hebrew was deeply encoded with different error checking mechanisms to ensure the quality of trasmission. (For example, the middle word or letter in a book was marked, the number of various letters was counted and noted in the margin and so forth.) If you look at pictures of old Hebrew MSS you will see this, so called "Massorah" all over the document. (At Monica's request here is a link to more information on this.)
Consequently, before the DSS were discovered the most notable manuscript of the Hebrew Bible was the Leningrad Codex, from the early 11th Century, and comparisons between Isaiah in this manuscript and the complete Isaiah MSS from the DSS showed a breathtakingly few number of variants. A few spelling changes, and grammar changes, but nothing at all substantial. That is after 1200 years of manual copyist transmission. So much for the game of telephone!
So really, there is very little to concern ourselves with in regards to the "reliability of the MT", the data seems to indicate that it contains something extremely close to the originals.
The LXX has some great uses though, for example, it is a great resource in understanding the contemporary usage of the Greek at the time of the New Testament. It is also a useful source of data, since the quotes in the NT from the OT are very frequently either verbatim or close to the text of the LXX.
One other matter, the DSS came on a tour to Chicago where I live a few years ago, and I had the opportunity to see them. If you are a Hebrew buff, I strongly recommend you try to get the opportunity to see them live. They are quite different in many respects from the texts you see in modern printed Bibles, and I think that we tend to get lulled into a false sense that they were printed in the original. Even seeing photographs doesn't quite prepare you for the real thing. (For example, the hand writing is tiny.)
The same is true for the Greek. When you have seen the Bodmer, even in photograph, all your arguments about punctuation, word spacing and paragraphs quickly fly out of the window. N&A along with the printed MT are very sanitized versions of the actual MSS that archeologists dig out of the ground.
I think "which is more reliable" is too imprecise a question. There are situations where the Dead Sea scrolls agree with the MT against LXX and vice-versa, and in some cases there's just no way to know which version is older. One more precise question is how often are the Qumran manuscripts close to MT and how often are they close to LXX. Wikipedia says that according to several experts (Peter Flynt and Emanuel Tov) roughly 60% are in the textual tradition behind MT and roughly 5% are in the textual tradition behind LXX.
Fraser Orr's answer is excellent and I only hope to supplement his excellent answer with a few thoughts regarding "reliability."
When asking a question like this, it's important to state your purpose. Why are you asking this question? The logic is such that they have reliable uses and applications within their own domains and we need to know the domain in which you're asking. As it stands, the question is akin to "Which is more reliable, a Biology textbook or an Entomology textbook?" They are related domains, but have different applications within any overlapping domains between them. Instead, start with what "reliable" means, what are you trying to do, and how you intend to "measure" reliability.
If you're trying to debate the relative merits as a source for translation then according to Fraser's answer, it looks as though the MT/DSS mss. would be better-suited for such a task.
However, if you're looking for the New Testament usage of the Old Testament, the LXX is probably more useful (Why is the Septuagint (LXX) significant?).
From a Biblical Studies perspective, they're both tools to be used in the process of exegesis. How you use them (and the reliability of the results that you receive) depends on the context in which you'd use them. After all, you wouldn't use a jackhammer to set a screw.