Is the WReX an STI? If not, I'd keep the Mustang. I don't ever have more than one passenger, and I'm not too keen on the pedestrian, secretary styling of the WReX, so the Mustang gets my vote. I do appreciate the technical specs, but I'm not that interested until they can package that drive train into the BRZ.
If the WReX is an STI, I might be inclined to make the trade as long as it didn't cost me anything... and I mean that. Not one dime. If you cant afford gas, you cant afford the trade. When my car was stock, I was getting 21-23mpg in mixed driving and better than 30 on the highway. Do you really think a WReX can do much better? Take your foot out of it once in a while
Keep in mind, the Mustang has a MUCH higher ceiling when it comes to mods and a few well planned suspension mods will make a Mustang competitive with the suby's awd and independent suspension.