I have a feeling that the next gen mustang is going to leave this gen feeling cold and alone. The next stang should be lighter, lower, and have a suspension set-up that's leagues better. If the 6cyl eco-boost gets in it? It will probably worth buying a v6 as well. paying into this body style will be a disappointment.
I almost feel similar about the Z. The Z is one of those cars right now that is over-priced in these model years. The suspension has been dumbed down for the track to make it a street queen. Nav and stereo options are way outdated and sub-par for the price.
The Z and mustang are both great cars but are ending a generation and both are probably going to have perfected next gen cars.
If you are hell-bent on getting a car now? I would strongly consider the EVO. and i would do it with the intention on paying it off withing two or 3 years. The STi is the slightly better value as far as build quality but the EVO is slightly more exciting to drive which make them essentially a tie.
But the Evo is halfway out the door. It will be a historically valuable car in the import tuner world. And a few grand spent at JP coachworks could enhance the Evo's weak interior. It's cheap to mod and easy to get 600rwhp out of. I know you have no plans of tracking but these qualities may be valuable to someone else when cars like this become hard to find.
So finally, when you pay off your Evo and have your classic and not so common mitsu car in your collection. I'd look at a Stang or Z. I have high hopes for both cars in the coming future. higher hopes for the stang only because at least there is info on the direction in which the stang is going.
but this is coming from an enthusiast perspective and NOT a DD perspective. DD wise, my choice is one of the worst choices and everyone else's advice is a little more sound.
But another perspective is always good to have.