They're marketed like mattresses, different selling channels get different model numbers/colors. Dolphin has two basic variants for their consumer market, the M-type (float handle) and S-type (water-jet propelled). There are different models within each family, generally offering more cleaning cycles/modes. Realistically, any of them will clean well. Some key differences are that the S-series has (in my opinion) the easiest-cleaning basket and is appreciably lighter. The M has more vigorous brushing and greater water filtration (not by much, 10-15% more). Like I said, all will clean just fine, some may need to run 2 cycles in certain circumstances. All in all, they are "set and frget" devices where you really don't need to be around. Anyone who has manually vacuumed a pool for any amount of time will tell you they're the greatest thing since the invention of the wheeled suitcase.
