The thing to note about socket bonuses is that they're not counted in the Item Level budget.
They are just that- a bonus! Specs that benefit from multiple statsb (tree: sp, haste, spirit, int) can go for the socket bonus for added performance
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If I do match gem colour, it's only with 9sta for 1 gem bonus, or higher. Trading 6 sta for 10 of something else can be worth it if you need it. However, with the 30% buff stamina scales really ridiculously.. But stam stacking isn't the answer to every fight. Just about 90% of them or so. Some of them want resists, expertise, block.. It really depends on the fight and you should keep extra gear with different gemming if you encounter these fights.
You need to be able to survive 2, if not 3, melee hits from the boss you are tanking, in a row. If you can't you need more sta. If you can give up 3-6 sta by putting a hybrid colour in a socket bonus for something you do need, that's how to get it.
As far as i am aware, there are only two slots of armor that will give you a +12 stam bonus - head and chest. On your chest use 3x 30 sta, but for head, go for a meta and multicolor gem - 15stam+12stam = 27 stam. You lose 3 stamina (30 health, or 33 with paladin stamina scaling (roughly), in exchange for a minute amount of dodge/parry or whatever you gem for (i go for parry - better then dodge, because you always do an extra auto-attack right after parry, meaning parry has a slight amount of additional threat value.)
It really doesnt make much difference, to be honest, but if your an elitist who will kill for an extra .01% of something (as some people are) then go for meta gem + 15 stam+10parry in head. If you cant, go for 15stam and 10 dodge rating.
Really doesnt matter though. Imho, go with whatever is cheaper and easier.
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