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.