Should I buy a lift with fixed (F) brackets or the rail (R) system?

If your platform is less than 24 inches (600 cm) long, you should buy an “F” or “fixed system” because the rails will not help balancing the weight on your boat. The dinghy is already as close to transom as possible. With platforms over 32 inches (900 mm) long, you can choose either one of them. The “R” or “rail system” is to get the weight of your PWC close to the transom so that the boats center of gravity does not move too far aft.