I prefer spell damage over intellect. I'm not saying intellect is not important; it is, but when you consider the frost tree is much more efficient mana wise than the other two for DPS, and that it doesn't get as much overall damage, I need to boost it as much as I can.
I would just go +spell damage myself, you're at 61, you're going to want to be able to kill mobs faster than to worry about your mana pool, which will only factor in when you instance, and even then, assuming you're grouping with understanding people, you'll have plenty of opportunity to regain mana in an instance. Also, don't forget that in the next patch, the mana gems are getting a nice overhaul; more mana restored and three uses per gem (even though I think this might be restricted to the Mana Emerald and the Mana Ruby).
If going +spell damage becames a big issue and you find yourself OOM way too much, then try going +int.
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Where is the point at which I pick +Int over +Damage? I am not sure how to do the calculation which is why I ask.
Probably never. You have to keep in mind that when you do begin raiding, you are going to have to have a decent +spell damage to effectively contribute to your group. The first round of Kara (Attunmen) a mage is there strictly for DPS support. You will want +spell damage.
Also for raiding, you will need +spell hit. This is critical in a boss fight. All the spell damage in the universe won't do a bit of good when RESIST keeps scrolling up.