Dragon Age seems much more suited to an MMO game imo. Couldn't see them being able to make a Mass Effect MMO that still retains all the elements of the single player games. The series has always been very cinematic and driven by the shooter elements. Not saying it's impossible, but it'd be difficult to transition the series to an MMO and have it still feel like Mass Effect.
I think Mass Effect has the most upside because it is different. It is a huge name that certainly carries some weight, and could introduce something new to MMOs, not just another mage+warrior+rogue+combos-of-the-three game with swords, staffs, bows, and shields