Personally, i'd get Mass Effect 2. Although, a better alternative might be to just rent both. Mass Effect 2 is just a big campaign, so once you're done, there's no multiplayer. You can replay it, but i haven't. Dead Space 2 has a really good campaign, from what i've heard. And i also heard the multiplayer was lack luster. So renting both might be a good idea.
But if you want to buy one of the two, it just depends on what you want. Do you want a really deep RPG which is a lot of fun? Go with Mass Effect 2.
Do you like scary games that are a bunch of fun to play? And maybe a little multiplayer? Go with Dead Space 2.