You have lot of variables to compare.
My Sprinter ClassB feels bigger than the first ClassC I used 30 years ago and feels more comfortable than Truck Camper we still have.
But feel is relative aspect and B has smaller holding tanks and can't beat storage on basement model of TC.
Sprinters also come in extended versions, where 18" adds a lot and I have seen B with slide.
So long way from converted Chevy van to extended Sprinter with slide.
My main reason for going from TC to B is more desire for fast travel since our sons spread in 2500 mile radius.
Compare 10 mpg on TC to 22 mpg on Sprinter.
600-800 miles between fuel stops.