My mage has the 40/0/21 spec, Arcane-Frost.
I currently have the +40 spelldamage enchant on my weapon. (http://www.wowhead.com/?spell=27975). This gives +40 spelldamage to all schools of magic.
On WoW-Heroes, this enchant is rated +8 for enchant while it is in fact, the best possible weapon enchant for my spec.
The enchants that would give me +10 would be Soulfrost (http://www.wowhead.com/?spell=27982) and Sunfire (http://www.wowhead.com/?spell=27981) which would respectively give +54 frost damage or +50 arcane damage to my spells.
As a typical Arcane-Frost rotation would be 3xAB+3xFB or 10xAB+2xFB depending on your mana-regen. All in all, in my opinion, a general +40 spelldamage would be more beneficial then the +50 arcane damage as you would lose 50 spelldamage on your frostbolts you're bound to cast at some point, unless you have a shadowpriest AND a shaman in your party. :p
Again, I don't think specs are being held in account (yet) and obviously for most cases the scoring would be correct. Just wanted to moan a bit.