THe AC outlets are generally higher. If you're trying to take the "chill off" or really heat a boat that's sitting in cold water with a blige that is ventilated and influenced by the water temperature, then there are real advantages to the dual systems.

Of course, if you're at a dock (where energy is, for your personal purposes, free) and the water and outside air temp is not to cold, then you can keep the boat really warmer -- then it may be "okay". It still won't be as comfortable.

If you're concerned with energy, using the heater intermittantly, etc. -- then, if you have both systems, it's a lot nicer.