Well, I haven't played fallout 3, but I did read reviews about it, and I got Mass Effect instead. So before I say anything, keep in mind that my opinion might be a bit biased towards Mass Effect.
Apparently Fallout 3 has a much longer gameplay and more to explore and everything remains interesting. Basically it gives you a lot of freedom to explore everything you want when you want to. The gameplay/battlesystem is very unique and has a lot of depth. From what I've read however, the story isn't that immersive and for me personally, that is really a huge drawback for an RPG. And replayability is not that great, but it doesn't need to be, since the main game is already so long.
Mass Effect is completely the opposite. The main story is short, the side quests are fun, but exploring other planets can be boring at times. You're free to explore them or not, and here and there, there are some real fun encounters on distant planets, but overall, it's basically travelling completely dead land. The battle system is not that interesting. It simply plays like a 3rd person shooter with some extra powers if you chose a different class. Though it's actually pretty deep from an RPG perspective, if you take all the armors and different ammunitions into account.. But setting the whole thing up might be a hassle because everything is basically organized in a single window. In any case, the most valuable part of this game is the story.. It's the first game, and the first time, that I actually buy the novels of a game. The story is so interesting and deep, and every alien species are not only different in looks, but also in culture and how they interact. They are so interesting.. And this is where replayability comes in. The choices you make in the game give you a totally different experience, and this game is built around the replay value, making up for the relatively short campaign. The emotional side of this game is also like no other.. There will be moments where you will not want to make a choice but have to..
In the end it's a matter of preference, but of course, I would choose Mass Effect.. Simply because it's not only a game, but it offers a very unique perspective on moral situations, and you can replay the game as much as you want, to see how things would have gone differently, without actually being bored by the length of the game, and it can make you aware of the consequences of your choices. Even though it's simplistic, it's a good reflection on how you would do things in real life.
Truth does not fear investigation
