A friend has an F550 with the fancy hauler package and it is very comfortable. Don't know if an International, e.g. the 4700, is as car-like, but I doubt it. They have some great seats available for the MDT's though - I'd get the best I could afford if going that route. OTR truckers always say that money spent on seats is well spent.
Medium duty trucks are developed from the commercial trailer tractor line of makers scaled down a bit. They tend to be built with high mileage (300K plus) long term heavy duty use in mind. Their ride comfort is based on towing heavy trailers with high pin weights. That said, like Gary said, a bit of investment in cab seating and goodies will pay off in both comfort and resale value.