RPGamer is right, choose what you feel comfortable with.
I check AC3D website and I couldn't find anything about
animations, but Milkshape don't have a rendering engine.
In my opinion you need ot ask yourself where you want this to go,
if you are talking about 1 (or 2) modeling package that
will take you all the way choose AC3D with
PaceMaker or
fragMOTION for animations,
but if you plan to buy new softwares in the future go with
milkshape becuase it's cheaper and has a lot of import/export
worry about those render engines and all other stuff in the future.