Can't really comment on what's in store for legion, but right now Sub is significantly better at ST. Mut's rotation is indeed easier and its damage output isn't terrible by any means, but if you really want to support your guild to the fullest, play Sub/Combat. Subtelty's main stat - Multistrike - is actually good for Combat when cleaving, so there is synergy between the specs that you can't really have with Mut.
Long story short, it's a question of your preference vs your guild's need.
If you want numbers, you can check the Simcraft results post that's been stickied:
http://www.mmo-champion.com/threads/...esults-for-WoD