Questions like these are why stat weighting scales were developed; they aim to answer questions like that by expressing how much each stat is worth in terms of "points," which quantify how much each stat helps your DPS/healing/tanking. For instance, Shadow Panther has calculated that if 1 Agi is worth 1 point, 1 hit rating is worth 0.85 points, 1 Str, 0.55 points, and so on. Many different rating scales for various classes and specs can be found around the internet.
But what if you want this information in game? Pawn, one of my favorite addons, solves that very problem. Simply feed it a weight scale (or more than one), and it will calculate and display item values on all your in-game tooltips, making comparisons a snap. Of course, these numbers are meaningless if you don't have a good scale to start with. Here are a few I've used and liked:
- Priest: Dwarf Priest for Discipline and Holy. MK has done a lot of math to put this together, and it shows. I still think Int is a bit high, though, and Spirit a bit low. For Shadow, ShadowPriest.com is the best game in town.
- Death Knight (DPS): Elitist Jerks. Note that Agi is not on that scale; I just calculated out how much crit it gave me (at whatever level I happened to be at the time, 63 or so) and rated it accordingly.
- Rogue: Shadow Panther. This site has been around approximately forever, and has scales for both PvE and PvP that are quite trusted.
- Druid (feral): Emmerald (Bear/Cat). The Wrath lists are still in beta, but a finalizing update is on its way.
It would be irresponsible at this point not to give a caveat: stat weightings are not gospel. Think about your gear choices, don't just blindly pick the one that ranks higher on some list. Hit rating, for instance, is useless once you're capped; I actually like to split Hit off to a separate scale in Pawn so I can take that into account (as seen in the screenshot above). Most non-tank scales do not include Stamina, as it doesn't raise your DPS/mana regen/healing throughput, but a dead character is not good for much, so Stam isn't really worthless; it's just harder to quantify.
Furthermore, for many (if not most) specs, it is ideal to balance between stats. Cat druids should seek a balance between crit and AP, for instance, and holy priests between throughput and regen. Rating scales don't really help you out with this, as they don't reflect that the value of stats changes based on your other stats. What will help you is simulators and spreadsheets, but that's a topic for another post. For now I'll just mention that Rawr has a simulator for many specs (including excellent feral ones), so give that a go if you're interested.
However, that said, Pawn is a great way to quickly help you decide if that quest reward is better than the one you're wearing, or if it's really worth it to get that item crafted. And the weight scales I linked above should help get you started. Does anyone else have some favorite stat weightings?
Edit: Several of you are asking how this compares to RatingBuster. RatingBuster is a fundamentally different mod, with different goals. It was originally invented to break down combat ratings: to tell you, for instance, what 10 hit rating translates to in a percentage. It has been expanded to do similar things for other stats (like telling you how much crit agi gives you). It will not help you decide whether the item with 10 agi is better than the one with 10 hit rating. As you may note in the screenshot, I use both; neither is a replacement for the other.