Physics engine choices? (for cloth sim)

I’m pretty new to OF and just starting to experiment a little.

Right now, I’m working on something that uses a cloth-like sim (i.e lots of particles connected with spring constraints). So far, I have tested MSAPhysics. While that worked nicely, I have found that the library is not parallelized, and I need pretty good performance for what I’m trying to do.

So, my question is: What are your experiences with the available physics libraries (MSAPhysics, ofxTraerPhysics, Box2D, several variants of Bullet, …)?
Which are parallelized (or even use the GPU)? Which make is easy to do something like a cloth sim?

I’m mostly looking for particles with constraints, attractors, maybe vector force fields. In this case, I don’t really care about rigid body dynamics or similar.