I've had neither system on my own boat. I did have a Force 10 kero bulkhead heater, but that got trashed.
I've had good experience with Espar hot air on a friends Cat 42 - turn it on, instant heat. Just what you need when you're hypothermic and your teeth are chattering.
Problem is running the ducts - they're big. A hot water system needs smaller pipes, but is slower to bring things up to comfort levels.
For live aboard, I think I'd go hydronic hot water. For occasional use, the hot air system is attractive.
Hopefully we'll get some input from someone using hot air for full time heating.