As far as you are concerned and detailers are concered there is no difference. The backing plate may be different size, but I would definitely go for the 7336 at your local Lowes and save yourself a fair amount of money.
I believe the counterweight on the 7424 is the 5" weight, and the 7336 has the 6". Since the pads most often used are 6.5"+, the 6" counterweight is the one used most often. So in that respect, the 7336 is the better one to buy.