Loki wrote:
Spell damage is roughly 1 threat per point increase, meaning unless you are putting out (i.e. easily sustaining) 1k TPS, you want the 20 spellpower enchant.
Wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong (sorry, had to say it a lot because it gets repeated a lot here)
I did the math, and posted it, you can review it and the references at http://wow.allakhazam.com/db/class.html?wclass=2;mid=1205162229156975173#m1205261840148029590
If you are doing 630 TPS or higher, +2% threat is a better enchant.
If you are doing under 630 TPS, you don't belong in a raid dungeon, especially considering the first raid dungeon (Karazhan) is packed full of mobs that we can blast with Exorcism for even higher than normal TPS.
The breaking point is not 1k TPS, or 500 spell damage. The reason +threat is so much better is because it increases all your threat. Spell damage doesn't impact things like: self-healing done from Earthshield or Prayer of Mending, Retribution Aura, Mana gains from Judgements and/or spiritual attunement (yes, that's right, people healing you does increase your threat), Fire Shield or Thorns, Shield Spikes, and more.
If you raid, you should have +2% threat. If you are still at the 5-man level, go for +20 spell damage.