The wheels on contact decks will leave "tire marks" in the turf. On softer ground, those wheels will dig like you wouldn't believe. They're supposed to caster/turn as the tractor turns, often times they get stuck, worn out, broken, etc and then your cut quality suffers drastically. Also, to adjust the cut height of a contact deck, you have to move the wheels/casters up or down, sometimes also needing adjustments on the lift linkages.
This is why suspended decks got popular. Less maintenance and cheaper to own over the life of the deck/mower. Typically, in this area, suspended deck will give an excellent cut quality with low maintenance. I haven't seen a single piece of flat turf here. There's bumps, ruts, rocks, etc and they all affect the cut quality negatively with contact decks.
Both have their pro's and con's, sometimes when you're looking at a belly deck, one or the other is all that's available.