You gotta do the math as to how many amp hours you will be consuming and producing. I googled the alt kits you mentioned. The one I saw looked like it put out about 7 amps. That would probably be enough to help power a laptop or small flat screen tv. As to the lights and heater, I would convert to led lights and a propane heater.
You don't say how you use your boat. If it's just weekend trips, I'd go for the kit. If it's a liveaboard, I'd spring for the honda generator. Also, have you checked that your outboard doesn't already charge the battery and doesn't need an add on kit?
Gemini Catamaran Split Decision