With sublety you can do hard switches, mutilate will do more sustained damage. Really up to you.
For example, with sub you could wail on someone and force them to blow cooldowns, blind them (with glyph), switch to new target with redirect kidney shot, archangel and mind spike them to death in the kidney, which would be very nice.
But, in reality, at the moment, arena is basically just "who can zerg who faster" at the moment and in that respect I guess mutilate is probably better - blow all your cooldowns with cold blood envenom and hope for the kill.