Barry,
I've seen the surface cleaners by Northern. Most are plastic and cannot withstand hot water. A surface cleaner without hot water is not half as usefull.
I'll bet the difference in GPM is in the spinner bearing and the tips.
Personally, I would never order a professional piece of equipment from Northern. I count on my surface cleaner to work, and work well. If I should have a problem, I want to be able to call Mark and get the parts I need to get it back and running. I don't want to deal with an order-taker at Northern.
It's fine to order some stuff from Northern, like less critical parts, or stuff you can't always get other places.
I know how it is to start out and have limited dollars, but certain things are best to not skimp on. Call Mark and at least check his prices before buying from Northern. You might be surprised.