I have the Pro Video II light. The head has both a modeling light and a 5500K color temperature 50W video flood integrated. The modeling light helps you to aim your head correctly before you flip on the flood. Also, having the modeling light integrated is handy during night dives. You can use it as a primary light so you don't need to carry a flashlight in one hand and the camera rig in the other. This is great. (Note you don't leave the flood on all the time... only when your shooting film.)
The single head won't cause backscatter if you aim the head correctly. However, it can cause shadows. I only find them noticable when night diving, but can usually compensate by adjusting the angle of the head. Eliminating shadows would be easier with two heads. The Pro Video light battery pack will support two light heads. It also supports battery pack stacking. Although I may add a second light head someday (one with a 50W flood and one with a 100W flood) it's not a priority at the moment. I shoot mostly close-ups and get good results with the single 50W head.
One other thing... leaving the flood on underwater at night will often attract lots of tiny critters (i.e. schools of tiny bait fish and worms) that can be a distraction from your primary subject. This will be a problem with any flood. To reduce this, I keep the flood of until I'm ready to film the shot.
If you wanted to spend big buck you could look into a High Intensity Discharge (HID) lighting system. I haven't researched them in depth because I'm happy with what I've got. If you go this route, just make sure the heads are floods and not spots, and that the light is the correct color temperature so your colors come out true.
Brian