Like any other equipment purchase.
Demo them all. If they don't offer a demo machine maybe they are hiding something.
Talk to other owners in your area. If the dealer won't tell you who the local owners are maybe you should look elsewhere.
Determine which local dealer has the best service and parts department. They all need service and repair at times.

Cost is seldom the driving force behind any purchase I make. Money spent well upfront will determine your eventual bottom line.

As far as diesel or gas goes, in most cases diesels will be more cost effective over the longhaul and will have better resale even if traded in every few years.

__________________
Of course this is only my humble but well thought out opinion.
Bill
Northern Turf Tech
Kalkaska, Michigan