For most routes you'll end up doing both with your chosen beau, although some routes let you pick just one, or don't have a choice at all. I may not have explored all the options, but here's the roles I've found for the chosen characters (I'm listing what the other guy does, not the main character):
!!!!SPOILERS!!!!
Spencer: versatile (your choices affect which position you take for your first time)
Dozer: versatile (you can choose to top Dozer on your first time, or to bottom, if you bottom you'll have the option to top him)
Chester: versatile (if you choose, otherwise he prefers to top)
Harold: bottom (if there's an option for Harold to top you, I've never found it, nor really looked to be honest)
Darius: versatile
Richard: ??? (No sex scenes have been released for him yet)