When I gave CRDTS, I did composite for Class II on a max molar because my patient wanted composite. She didn't want amalgam. I did it for her but if I had a choice I would do amalgam if its a huge lesion b/c you can condense better with amalgam then composite. I mean you can contour better with it too. I hope this helps.
P.S. If you have a choice do Class II first then Class III