For that kind of use they want something that is reliable out of the box, and supported. Not something that they have to shim and fuss with to get working right. I have been very happy with Oregon support on their products. I can call an 800-number and always speak to someone live. They send out parts very quickly if needed.
A lot of guys will comment here on A.S. that their clone was '
cheaper', but you don't hear them bragging that it is '
better'.
I played around with the cheap, Harbor Freight type grinders in another thread, just for fun, as a chain geek. But I would never recommend those for heavy, or professional use.
Oregon replaced their entire grinder line this year. There is a new model that replaced the 511A, and 511AX. They went back to the basic vise - several people had issues with wear with high-volume use.
http://www.arboristsite.com/community/threads/new-oregon-grinders-2015.270287/
Some if the Silveys have great reputations, but they are NLA. And the STIHL grinders are almost twice as much.
So I would recommend one (or two!) of the top of the line Oregon grinders (model 520 or 620). A lot of higher volume sharpeners like a second grinder so that they don't have to switch wheels, or so that 2 guys can sharpen during down time.
Philbert