Didn't we have somebody asking this exact same question, with a poll, a little awhile ago?
I couldn't find it, but I did find this:
http://www.escapistmagazine.com/articles/view/editorials/reviews/5419-Review-Fallout-3
http://www.escapistmagazine.com/videos/view/zero-punctuation/48-Grand-Theft-Auto-IV
There's escapist reviews of both games (at least if you call what Yahtzee does to games "reviewing" them) and they're sort of there to give you an idea which you want to buy, so read up.
My personal answer would be the same I give to anyone asking "durr, how should I spend my money" advice, and that is to
give us a clue as to the kinds of games you enjoy. If you're not going to do that, you might as well just throw your money down the sewer for all the enjoyment you're going to get out of it, because everybody enjoys different things and none of us are mind readers.
If you decide to go with the sewer option, might I recommend my PayPal donation button instead?