Alright sounds like you've already made your decision but I'll give you my opinion anyways. I own both, and recommend that you eventually buy both as well. Go to ebay, and you can find the GOTY edition of Morrowind for less than twenty dollars including shipping in most cases and at game stores near where I live I see copies of it all the time for under fifteen bucks.
I really enjoy both games but I rarely pick up either anymore...the reason being I've played them for so many hours there's just nothing I can do that I haven't already done. I believe both have alot of value in gameplay because you can play them for days without ever finishing
everything there is to do. As for length, Morrowind is much longer and takes a long time to finish everything there is to do, while Fallout 3 is slightly shorter.
Fallout 3 is more action oriented, while Morrowind is more roleplay oriented. Story wise, Morrowind seems much more satisfying than fallout 3 for me at least. But combat in Fallout 3 is much better. Dialogue is all done via text in Morrowind, which may take some getting used to for some...and Fallout 3 has full spoken dialogue. Graphically, Fallout 3 wins hands down, but it has the advantage of being several years younger than Morrowind as well. Finally, if you're looking for realism, you won't like either game because they're both very abstract in most aspects.