Allow me to echo the comments made by others. Your thesis is an important educational and career milestone.
I agree with
MisterMe... it's a thesis, and one should focus on the science and art (or whatever else is involved) and not the software. Unless it involved a lot of formulas and equations (in which case LaTeX would get more and more favorable), or one actually didn't know how to use Word, I don't see a good reason not to use Word.
As a separate argument, theses (and dissertations) aren't that long or that cumbersome... fancier software for long writing projects gets more and more useful as these factors increase in the equation. I think it's different when something is going to be 300-400 pages or longer, than the 50-150 pages a thesis or dissertation is going to use.
Finally, as an addition, if you're going to pick up a tool, you might also consider instead picking up EndNote instead. It's a package that works in concert with Word and other apps to manage references. It can do a lot of useful things like translating a citation between different cite formats (e.g., end notes vs. encapsulated references, different style guides' ways of formatting references, etc.). Now I didn't personally use this software at all (for either my thesis or my dissertation), but my friends who put the initial investment into swear by it. The nice thing, too, is that it becomes more and more valuable over time, since your citations stay in it and remain available to use in future journal articles, etc. There's also a similar OSS app, although it doesn't seem to do as much.