I may be a Siren, just because I think they'll be in high demand for all co-op parties.
I actually may start a siren and a commando and try to keep them at similar levels so I can go back and forth between the two.
Hopefully they will eventually release DLC that will increase the level cap. As it is now, you can only fully max out one skill tree, and that's not even maxing out every skill, just doing enough at each level to get to the augment skill at the end of the tree. They would have to triple the level cap for you to be able to fully max out.