Matt, you'll get better flow rate from the FX5, but the tubing, being much larger, presents some problems hooking it up to standard equipment. So for example, if you want to use a spraybar then it has to be custom made to fit the hose. If you want to use an external heater, there are none with this hose diameter so you will restrict flow trying to fit an adapter. As far as I can tell, the only generic problem with the pro III is the idiotic electronic enabling. That's why you need to get the 2180. They are dumbed down and solid.
Cheers,