My choices are the same whether you are a tank or not. JC/BS, awesome gems (and epic gems in the future) and more gems. If you are a tank you can get hp, you can get AP, Str, Agi, Spell power, crit, hit, whatever.

The reason why 3 prismatic gems are awesome, apart form being awesome gems, is because it basically allows you to gem the rest of your gear how you want and you will still make your meta gem work.