Think gas, reliability, workability, driveability, and luxurability (?, i tried). Supra has these in the bag in my opinion. Way better engineered, way more functionality both in the engine and the car's features. Better on gas, and you know it will last forever, depending how you treat it of course (as that is just how toyota's seem to be in my experience). Both can make huge numbers just not nearly as much torque for the supra. They both look great, and personally there are way too many mustangs on the road as it is, go a bit more unique
Edit: don't forget resale value!!!, Supra dominates this category with a 97 with 80K KBB is saying 32,000! off the lot it was about 39-40000. Add some good mods in there and its value goes up like the horsepower itself