I'll leave the re-sale bit alone, I have the onboard HWS and shower and have a mate with Joolca freestanding i.e. not plumbed to the camper.
Good things for onboard - no set up time, hot water to the sink
Bad things - only uses onboard water tank and the shower output is not great. Lose a cupboard and shower is right next to camper
Good things for Joolca - better flow and temperature control, can use alternate water sources like river or from a bucket/jerry to save onboard water for drinking. Can also set up further away from camper for run off control
Bad things for Joolca - set up time, theft