I have both. They're both really great games, but they are completely different from each other.
Dragon Age has a very Lord of the Rings feel to it. It's a really long game. The battle system reminds me of Final Fantasy 12 a little. It's real time battles, but they have a very turn based feel to them. It's also not the greatest looking game, and the menus are a little clunky.
Final Fantasy 13 has a very sci-fi feel to it. It's also a pretty long game, but it's very linear. You pretty much move forward through the game, with no freedom to explore. The game does open up more towards the end. The battle system is really fast paced, but there is still strategic elements to it. Also the menues are really sleek and intuitive.
I'd like to ask a follow-up question. I just bought Dragon Age but now I'm not sure it's the right game for me. I like action RPG's such as GOW III or Castlevania and on the other hand hated FF XIII (don't want to hurt any feelings, just not my kind of game), could anyone tell me which kind of game is Dragon Age closer (GOW III/Castlevania or FFXIII)? Thanks in advanced for any help!!!
Dragon Age is closer in play style to Final Fantasy, in that it's an RPG, you have to utilize the menus and at least minimum strategy/tactics for your party in order to finish the entire game without killing yourself or destroying your console in frustration. :) Castlevania and GOW are more action-adventure with a few menu elements thrown in to give the gameplay a deeper element.