do you need a blue to make your meta work or are you just going for a bonus? if you need a blue then I'd do the stam/agi. straight stam is kinda bunk since you can't stam an enemy to death. if you don't need the blue for the meta then stuff a red one in and call it good.
Go with the 4agi/6sta no matter what. The socket bonus is 4 agi. So by putting in a 4agi/6sta you get 8agi/6sta. By putting in a red gem you'll only get 8agi or 16ap...