I believe it matters on your game. Any of the physics engine's are fine if you want basic physics such as mass/gravity and so on... Currently I am using 3dgs PE in my game, the only major problem I have as of now is that the wheels bend when I reach high altitude, and turn... Marco said he will fix that eventually though. I noticed that you want rag-doll physics, I am unsure if Newtons physics engine can handle this... but it would be quite hard in 3dgs PE. I guess it all comes down to what you can afford, Havok...mega bucks, 3dgs PE... pro version, Newton...Free, also compatability is an issue too.