I'm beginning to prefer WhiteCore (formerly Aurorasim), though there are -of course- stability problems.
Varregions is nifty now that I have a feel for it, but BulletSim is just too, too resource intensive.
Mega-regions I don't seem to be able to make work for love or money.
Of course, the scripts I have tend to work the best on opensim/ODE -but running around a 256m is no fun at all.
This is just my personal experience, of course.