Precision

For PvP specced casters :
Now that the last % of chances to miss is removable, capping hit is something every serious PvP player should do. To reach the PvP cap of 4%, or 105 hit rating, i would recommand to take http://www.wowhead.com/?spell=44488 and http://www.wowhead.com/?spell=60623, plus the http://www.wowhead.com/?item=44416. That will give you 98 hit rating, which is not enough. Complete it with a hit gem (that gives at least +9, we'll see why later) and you're capped. Note that if you're draenei, or have a draenei teammate, you have 1% hit offered by racial. That puts the cap at 78, so you can remove the glove enchant and the gem (note : you may miss sometimes, because it's actually ~78.6).

From there, you will be hit capped, and should not miss spells anymore. However, keep in mind that your spells can still be resisted. As armors can't be swapped in arenas, the best choice is to take the spell penetration weapons. That means you will need 4 more http://www.wowhead.com/?item=40014 (don't forget the +9 hit gem from the first part!), and could take a crit/haste weapon apart from the penetration one (and instead of the hit one btw). But to do that, you need lots of arena and honor points, so i don't think it's a good choice if you're gearing up. Not all classes got spell resistances (I think only mages do, now that the master demonologist talent has been modified).

As a tankadin I would say the precision enchant is to be favored over armsman, since we are not having any threat problems, (from my experience). However I'm at 215 hit rating in full prot gear atm and I'm still having quite alot of resists (misses), which apart from lowering your damage output also lowers threat caused.

Itemization wise this enchant is worth more than 15 expertise to gloves. This is the better option for Tanks unless you are way below the dodge cap. (6.5%)

This enchant will be cheaper starting tomorrow with the launch of 3.0.8. (January 20th)

It will now be 4 Greater Cosmic Essence.