Hi Dashing & welcome to the forum.
Plumbers offering advice on wet-room construction can be a real hit or miss calculation you should make before committing to installation.
Is he competent of installing a wet room and has he a proven track record of installations previous.
Most plumbers I know should stay clear of wet room installs and without experience or wet room qualifications, should not be quoting for these jobs anyway.
In my experience most plumbers are not capable of installing wet rooms.
Right! back to your question.
If your "Plumber" has given you a choice between Marmox or moisture boards I hope your talking about wall preparation and in my opinion Marmox is superior!! However, Moisture board installed with a good tanking kit like Ardex will work fine and also should be more cost effective.
I would also recommend you should Incorporate UFH with all wetroom installations and your tiling is carried out by a competent fitter.
I honestly don't mean to give Plumbers a bad name in fitting wet rooms as I am also a plumber, but trained in Impey wet room installations, and has a proven track record to back it up.
Sorry if I give you earache in this plumber bashing and wish you well with your install.