To see where Monster Hunter series sells better, they should release game for both platforms at same time (Monster Hunter 5 for example).
Saying, that MH sold better on PSP or was better success than on 3DS is irrelevant.
Still I think, that bringing MH back to Sony will hugely increase PS Vita sells. Seriously people in Japan are waiting for Monster Hunter for Vita (don't tell me that Monster Hunter Frontier G is equivalent for MH 4 for 3DS)