I know this is an old thread, bit I'll add my 2 cents to the original question incase any future readers make it this far in this thread.
First....there is little difference between most of the big name commercial models. They all have pretty much the same engine and hydro options. Is unless you are fond of a particular color... Go in with an unbiased mindset and look at the ergonomics of the mower and see which one fits you best.
Regarding size....all depends on the lawns you mow. If you are mowing 2+ acre country lawns, the bigger 60"-72" machines are nice. If you are mowing 5000 sq ft town lawns those big mowers are cumbersome, more expensive, and sometimes just slower.
Style would be who is operating. If you are and owner operator, get a rider. If you are having employees work....stand-ons like scag v-ride or toro grandstand....and the walk behind models with sulkeys. Most mow companies use these because if you put employees on riders....they won't get off. They won't pick up trash, sticks, rocks, dog toys, etc. Which leads to less happy customers and higher maintenance costs (more frequent blade sharpening, changing, and less spindle life).
For engines, I much prefer the Kawasaki to any Kohler.