Tutorial link: http://blog.askmrrobot.com/sim-items/
You can compare 2 items very quickly using simple simulations on our site. There is no need to download anything or fuss with item IDs. Just use our item picker and you’re all set! You can even easily edit the item levels in case you want to look at a warforged version.
This tutorial covers the quickest way to compare 2 items if you want to use the simulator. There’s a 2-minute video that demonstrates just how fast and easy it is (it takes less than a minute to setup, but the video takes long so as to explain things more thoroughly).