it depends on what you looking for. the wtx sti will off-road a little better than the mitsubishi and trust me i know this for a fact also the subaru is cheaper i believe but in not sure. both great cars. its just personal preference which one you buy.
Both are awesome cars, and I think both have the same goal, being practical and FAST, but I think the "WRX STi" is better, notice the "STi" but not the "WRX"
I very nice way to find out which one is best in performance topic, is to take advantage of a car performance simulator, there you can set up this (or any) car in the world, and run performance tests like 0-60, drag, top speed, and so on, you can even set up your weight as driver plus tons of features