Can't be too sure since it's unfeasible that both sides have enough experience in both. I used to think Mains, in particular those with did RSB (Max Gear CWA), were the best of mains mechanically, but I've seen enough of them do wilderness, 55 def, and F2P to now think otherwise. Just as I've seen numerous pures on their mains that splash all over the place.
If someone spends all their time doing one kind of PvP, they're going to get good at it. Although I think I can say that there are more mains who suck at PvP than pures (on a % basis).