I'm guessing your barb must already be high level because you can get the blessed aim merc in normal as well as hell.
I had a similar post on the forum regarding mercs and equipment. I will be using the IK set and WW. They told me that I don't need blessed aim because the AR from the IK set is good enough and that the might merc is a better choice. So, I guess there is your answer.
Might for sure, there really isnt an alternative unless you have absolutely zero AR(IK takes care of that), or you strive to be different. But, yes, go with might, more damage=more leech for you and merc
I think every barb (but a singer) wants a might merc. damage is king as we say.
if your build is deficient in other areas such as attack rating, there are other ways to remediate that such as charms, eth runes, points in mastery (if not maxed already) etc. nothing beats just leveling up your barb though. the thing that makes the biggest difference in attack rating is character level vs monster level. if a monster is level 70 and you are 60, you will have a hard time, but if the situation were reversed, things would be much easier.
defense has taken a higer priority in 1.10 but i don't see the need for it to be particuarly emphasized in pvm. barbs are typically not lacking in defense anyhow due to shout (and iron skin). pvp is another story.
holy freeze is nice but i wouldn't rate it higher than might for any barb build (again, except perhaps a singer). if you can afford it, the doom runeword which can be used by a merc, provides the holy freeze aura so it is possible to have the best of both worlds.
and then thorns and prayer have essentially no use, particularly for barbs.
go with might! make sure you get the offensive one when on act 2 nm.
as for leveling them, check this out: http://strategy.diabloii.net/news.php?id=512. i've found it applies to mercs too. however, i often just take my low level merc with me on easy magic find runs (pindle etc) and he levels fine from doing that.