PS3 - big screen, other more immersive/demanding games you could be playing, initial gaming time on ps3
Vita - play it anywhere, fast to get into with standby, gameplay seems to suit a handheld well and it likely won't be missing much if anything going by how it looks, vita should handle it all.
Is it cross-play?
Definite Vita buy for me but then I can't be bothered for PS3 much these days unless it's a triple A experience to get me to sit down and lock in for it