Both - but choosing between the two Tarkin is an easy choice. His ship is elite and in arena he's also elite. Snow has no ship and in arena he's usable, but far from elite. Tarkin also has a great TM reduction skill that is $$$ in both raids (makes Rancor a joke especially with Thrawn and can solo p3 Haat with him as well). Back to arena - as we know speed is often so important - he has a +28/30 speed leader skill that is very viable. I have both g11 7* and love Snow, but this really isn't a contest if you're choosing. Tarkin is elite in every aspect of the game.
