Pawn is an addon that integrates in your tooltips, you can give values to certain stats and this will be used to calculate an itemscore for an item. A basic set of values can be downloaded from the official addon forum.

I personally use ratingbuster and multi-tooltip addon to see what item is better.

pawn - http://wowui.worldofwar.net/?p=mod&m=3999
elemental scale - http://pawnmod.trenchrats.com/viewtopic.php?t=216