Final Fantasy XIII is only an RPG in name. It is more linear than Action Adventure games on the N64. Honestly, I felt more freedom playing Zelda: Ocarina of Time recently. It isn't really an RPG to be honest. There is an inventory system and numerous characters to choose from, that's about all that is RPGish in the game. Having said that Final Fantasy is a decent game. Didn't grab my interest enough to make me keep the game, however.
Dragon Age is exactly what you would expect from an RPG. You create your own character and you go through the world how you want to. Having said that the game can plod at points and it follows very close to normal lore (Think elves, dwarves, evil orcish baddies) with a few wrinkles thrown in. It has much more replayability and is overall very fun with very strong characters. (Shale, Sten, and Morrigan)
As for playtime of both games, FFXIII clocks in around 65 hours for the main quest line doing some "side quests", I believe around 75 hours for total completion at my pace. Taking the same pace with Dragon Age I would complete it in around 40-45 hours completing everything. Initially FFXIII is longer, however, I have logged well over 150 hours on Dragon Age over 4 playthroughs. I got much more bang for my buck out of Dragon Age than FFXIII.
Overall, regardless of platform I recommend Dragon Age. It will stick with you much more after you play it, because well, it's your story. Don't get me wrong Final Fantasy is not a bad game. I just enjoyed Dragon Age much, much more. In fact, I enjoyed the expansion for Dragon Age more than I enjoyed Final Fantasy XIII. Obviously, this is my opinion, take it for what it is, my honest assessment of both games.
Overall Ratings I would give to both games
FFXIII-8.0
Dragon Age-8.5
(Awakening Expansion-8.0)