http://www.harborfreight.com/6-inch-buffer-94393.html
I prefer that one sinc eht earm is slightly longer so you have more radius to work with on oddly shaped stuff.
IME the 1/2hp is fine. Sure 3/4hp may be nice but I would want the longer arm.
1/2hp does not really bog down if you dont go crazy and press to hard (not how you are supposed to buff anyway).
The green compound is my go to for the sewn wheel and I use the next step down (white?) on the loose wheel for finr tuning.