In two words; Socket Bonus. Your want to get the maximum useful socket bonus when you are socketing for your meta because most likely the gems for your meta requires aren't 'the best' gems to be using.
Take for example your helmet; For an extra 9 spell power you need to put in a meta and a blue gem, the blue gem your using is a Purified Dreadstone which gives spirit. While spirit gives some crit due to molten armor and a bit of mp5 it isn't that useful for a Mage. The 9 spell power bonus though makes it worth while, 12 + 9 = 21, while not as good as a Runed Cardinal Ruby It helps keep your meta active.By this logic you will want to use your blue gem to get the maximum possible socket bouns. You would go with your chest (I'll edit the original post for clarity it was really early when I made my post and though 5 and 7 were the same number). You could use your belt too for the blue gem since it has the 7 spell power bonus, and your most likely won't replace it soon.
[Today 06:48 PM] Ion:swimming in a natural body of water ISN'T acceptable...it's momentarily tolerable