I don't know a thing about the Heatmaster. I do know that the Pro Fab is an awesome unit. I don't know of too many OWB manufacturers out there that have been making them(gassers) longer than Pro Fab. They've had plenty of time to work the bugs out of them.
Water capacity is not an issue with a gasser, as they need to cycle frequently to keep the refractory hot so they can ignite the smoke instantly every time the system calls for heat. mgw44 has a Pro Fab (Empyre) 100 and the boiler capacity on it is only 35 gallons. It's the same unit I had, and the only thing I would have changed on that little guy would have been the primary burn chamber capacity. They list them at a little over 5 cu. ft. I might still be using it if the firebox would have been 50% bigger.
The Pro Fab's also use the Scotch Marine style of heat transfer(same as Portage and Main), which has been around for over 150 years, and is a tried, true, and proven efficient means to transfer heat.
The only flaw that I can see with the Pro Fab Boiler, and this is just my opinion here, might be the area in the firebox that is jacketed by water. I believe it's only 8 gauge (about .165). I like to see them at 1/4"(.250).