This is something the knifemakers would be able to answer best. From what I understand, serrated wheels are supposed to be better for roughing. They are also supposed to run cooler, extend belt life and track better. I know a custom knifemaker and he tells me that he prefers a smooth belt because it finishes better and he has no trouble with tracking or belt life. He also told me that KMG, the machine he uses, no longer sells serrated contact wheels because they create a coarser finish.
I don't know enough about this to be helpful but I can say this. A smooth 8" contact wheel on a KMG will flat out eat steel amazingly fast!
All belt sanders need a platen. Not all have a contact wheel.