I vote for bullet, because it start lagging only on 3000 boxes and isn't 20mb size. Newer PhysX. Personaly, I hate when with 5mb game comes 20mb driver. It's quite annoying. Newton 2 is no need because it's easy used by plugin. Waiting for Microsoft to release some physics - <lol>, have you seen their Microsoft Physics Illustrator? It's poor and lagged.