There isn't much more to the question. I'm not concerned about overhead, as I'm sure they are both fine for my purposes. Basically, I am familiar with Box2D concepts because of the Farseer Physics Engine, but I want to use Bullet when I make the jump to 3D stuff. Perhaps Bullet has some educational value for me even in the 2D realm?
The generalized version of the question is: should I use a 3D physics engine for a 2D game if I plan to utilize a 3D physics engine in the future? Or is this a waste of time which would not provide educational value?