Supplementals usually pay more than regionals. You also usually get ETOPS experience.
At the regionals, you get the benefit of working with mainline dispatchers and dispatch managers if you work your way into coordinator or supervisory roles. They are run very similiar to the mainlines and tend to be easy to train due to system familiarity so a large percentage of mainline new hires come from regionals.
I would recommend going supplemental if you get offers from both. Mainline hiring is on the back end of a wave. I would plan on at least five years before getting hired by a mainline carrier. Unless you live in a city where a regional is based, go for the higher supplemental money.