The "Professional Use" qualifier helps.

Wet-Dry vacs are priced from $30 to $600 and for professional use, the smaller, cheaper models wouldn't be my choice.
Ridgid, Sears Craftsman and Shop Vac are three that I have owned and all three brands did what they were advertised to do.
Just my thoughts here:
1. Suction is a lot more important than tank size.
2. A unit that is stable when you end up pulling it around by the hose, (And you will

), is a good idea.
3. I presently have 2 five HP units and wouldn't want to have any less power.
4. If I were in the business, the 6.5 HP would probably be my low end want. (Unless you are mobil)
5. If you are mobil, I would look at the Ridgid Tool Box unit that stores the hose and tools in the case. It is very handy if you don't have a fixed location.
6. The difference in price for a unit with more power isn't that great and considering you might use it for 10+ years, that difference could be a good investment. The 5 HP units are usually in the $60 to $80 range and the 6.5 HP units are around $100.
The Ridgid Toolbox 5 HP was close to $100 at the time I bought mine, but I think they are a little cheaper now. It is handy, but if you are in a fixed location some of the others would probably be a better choice.