We are currently using both , simply because we had a webasto hot air unit already fitted , and when LBBY fitted our new front berth we had a Eberspacher D4 hydronic fitted with separate stats and matrixblowers for main cabin galley and berth.
The hot air unit was fast , noisy and smelly on start up , but great heat output that was uncontrollable and vanished seconds after shut down
The new hydronic is very controllable, heats quickly , heats the water , and almost silent with no odours . It can heat water only by press of a switch in the summer
Because it is sized to run about 3/4 to full on ave use it burns clean with much less coking etc
If the hot air ever wont start its gone , hydronic is deffo the way to go
Alex