newton has a lot more features (for example a lot more collision primitives), is a bit faster and most importantly it's robust.

with a6's physics you have to try hard to not get explosions (and it's not possible to get it fully reliable), with newton you have to try hard to cause explosions. (with explosions i mean entities which unwantedly rocket around.)

but a6's physics may be less overwhelming scripting-wise for a beginner.