We have a simulator that you can use to test specific items -- it is much more full-featured that simulationcraft and generally more accurate.
It is available via the main menu, or go here: http://www.askmrrobot.com/wow/simulator/run
It is very easy to try on different items and simulate it. There are a ton of options for you to play with under "more options", the most important though are setting your consumable options (potions, food, flask) as you like.
With regards to your specific goal, we have added a new feature called The Blender. It allows mixing toughness and DPS for tank gearing, specifically. We have some UI changes to it coming very soon, but for now you can try it out be creating a custom gearing strategy, or saving a copy of one of our defaults. More details here: http://forums.askmrrobot.com/t/the-blender-dynamic-tank-gear-ranking/941