Sensei on 10 February 2013
PS Vita: Persona 4 Golden and sign up for PS+ for some variety while you wait for Killzone Mercenary. Vita has many others shooters but I don't play them so I can't recommend any other than Uncharted.
For the 3DS I'd be tempted to get Fire Emblem but I would guess Vita has more shooters.
Why not get both.
