First thank you for asking me this, I love answering game related questions.
Alright so Fallout New Vegas (FNV for now on) is an open world game with plenty to do. I would argue that it has more content than Fallout 3 and was done pretty well though fallout 3 did do some things better. The game is not without its faults, it offers a wide expanse called the Mojave wasteland, for you to explore like Fallout 3 and Elder scrolls before it.
Dragon Age 2 while being an exceptional game kind of lacks a solid goal as Yahtzee said in his review of the game. Found here: http://www.escapistmagazine.com/videos/view/zero-p...
Both games though are good so what makes Fallout more worthy of you hard earned dollars? Well it really in my opinion comes down to which mood you are in. If you want an open world action rpg set in a post apocalypse world with lasers, plasma weapons, interesting characters and some insight on human nature, then Fallout is your game. If you are interested in a Fantasy game with dragons, action, and romance then I would recommend Dragon Age 2.
If you haven’t already gotten Fallout 3 Game of the Year Edition you could probably find that cheap online and it does come with all the expansions from the game where as FNV is still releasing its expansions. If you do get New Vegas I recommend picking up a strat guide to find all the good stuff.
Though a game you should check out is Rage coming out in September. That game is shaping up to look sweet.