You pretty much have 3 options - act 5 barb, act 2 might, or act 2 defiance.
The act 5 barb isn't a good decision because, unless you have a very good weapon (ebotd), the act 2 might merc will do more dmg and tank just as well. Then, you just have to decide between might and defiance. They will both tank just as well because the defiance merc won't get hit, and the might merc will leech more. What it comes down to is: do you want a higher defence (the sorc itself) or a merc that does more damage. I would take the higher damage merc.
*note that this is all imo*