Dragon Age is much more, for want of a better word, "traditional" rpg, in that you get an inventory and upgrade your armor and weapons etc constantly. That and the leveling up has a lot more to it.
Mass Effect 2, is much more of an all out Action RPG. Very little in the way of inventory, and you don't REALLY upgrade your guns and armor in the normal sense.
Both have great story lines (Personally I would say Mass Effect has the better story, but you would ideally want ME 1 first)
End of the day I would say, what do you prefer, fantasy, or Sci Fi, and make your decision on that.