Living aboard in Portugal. We probably use 40 to 60 amp hours/day, which includes refrigeration. The 20 amp Xantrex had no trouble at all keeping up, I don't think we saw over 10 amps of charge except the few times we lost power on the dock for half a day. We had an electric space heater but that was running directly off the 120 volts (we have an isolation transformer that steps the 240 euro volts down to 120, still at 50 cycles, but the charger is built for that and the heater didn't seem to mind.
I would go with the smaller unit unless you have lots of boat $ burning a hole in your pocket. LOL
The other issue might be the amps of shore power available for both the heater and the charger. Don't know if this would be a problem.