Fewest fights in HFC currently favor Sub over the other 2 specs. Especially Archimonde, since you have to switch targets a lot, Mutilate has advantages there, and you have loads of executes during the fight ( Deathcaller / Doomfire / Nether banish add / Infernals ).
Only two fights I would go Sub are just the pure single target fights, but the difference, is...
Well, lets put it this way, you should play mutilate on the last boss, and you should play combat on aoe fights, that will put you in a position, where you need to visit your trainer often