I would not recommend the 2500 with any fifth wheel, except the very lightest ones. You would only have the cargo capacity for a travel trailer with a 2500 truck.
Get the 3500 truck, if you think the pin weight will be less than 2,600 pounds dry, or 3,200 ready to roll down the road. Yes the pin weight goes way up when you fill it with camping gear and 60 - 80 gallons of water.
If you want a really nice fifth wheel, such as Grand Teton, then a dually is almost a requirement, and sure makes it more stable.
Air ride makes it nice when not towing, to soften the suspension. With all that extra weight back there, traditional springs will soften out a bit while towing, but sightseeing without the trailer can be a bit rough, until you let out some air!
Have fun with the new truck!
Fred.