Both are viable but sublety is superior, especially at higher rating. Also sublety is more fun. Mutilate mobility feels gimped when your sprints are gone. Yes, as multilate you can zerg down plate classes like warr/pal or mail like ele shamans. But every hunter or mage with a brain will just /lol at you.
With bis gear in 284 meta sub is alot more burst dmg. Mutilate is consitent dmg IF you can stick your targets, but smart players know how to counter it. They will kite and let your poisons fall off. Example vs warr/pal in 2s when you are sticking the warr and raping him with envenom spam, when your poison is about to fall he will call for freedom-intervene-kite, poisons will fall which means a complete reset for you.
If you are taking 1v1 situations and duels, sublety is superior in every way, there are so much more options with this spec.
If you still wanna try mutilate go with 1,8 / 1,8 dagger. deadly on mainhand / wound offhand.