[Maya or 3Ds Max to make SL avatars?]

i Find that Zbrush is best. Not only is it that well known, game designers use zbrush for models/landscape and the fact that its quite easy to use (works best with a tablet).

you can sculpt the model in zbrush and transfer it into maya for some rigging  or just make the model in Maya c: (3d max is also a good program but i don't use it so i have no clue of it)

or if you cant afford the programs you could always use blender