There's probably a bigger chance that P5 will come out on the Vita than the 3DS. Much like a new SMT title coming to 3DS than the Vita. This situation is sort of like KH (spin-offs on Nintendo, main-line and adding in a few spin-offs on Sony).
Seriously, the "Persona" appearing on the Nintendo conference means nothing. See "Shin Megami Tensei" for PS3.
I mean...I think a mainline Persona would do well on the 3DS. I think it could sell somewhere near 400k, which would be amazing for Atlus. But I really think they just have this loyalty to Sony for some reason. And I'm okay with that, since Nintendo gets Etrian Odyssey, Shin Megami Tensei, and a few quirky little games (Radiant Historia!).