I wouldn't trade my Mitsubishi Lancer Evolution X MR for either one of them.
That said, though, they're both great cars. However, if I were going to buy one I'd be tempted to go with the Camaro, whose motor isn't prone to weird ticking noises from the lifters and whose gearbox doesn't come from China. Besides, I like the Z-51 suspension set and the Camaro has better brakes. I do like the Mustang equally well as a car, though.
A little explanation for my choice of "Camaro" over "Mustang" in this situation. Take a look at my Evolution. It is an MR, true. But all MRs are not created equally. Most MRs are the "MR-Touring" version, loaded down with a really nice high-end stereo with big subwoofer in the trunk, full leather, a sunroof (which deletes the aluminum roof in favor of steel) and lots of sound-deadening insulation. A very small number of MRs are the so-called "base" MR, actually intended for club racing. They are actually lighter than a GSR, with no leather, no 9-speaker stereo, no sound-deadening insulation to speak of, and no sunroof. But they do have the two-piece, 14" Brembo brakes up front, the Bilstein/Eibach coilover shock setup, the forged aluminum BBS wheels, and the Getrag dual-clutch gearbox from the luxury-touring-minded MR-T. I own one of these. It's loud-you can hear the tires whine on the road and sometimes the hands free phone system (standard on all the Lancers now) can't hear me because of it. The exhaust BOOOOOOOMS when it's downshifiting (either by me or by the computer, I have my choice of manual shifting with the stick or paddles, or letting the computer do the work), and the suspension is stiffer than your average BMW-M series. And it was much too expensive.
But it sticks to the road like GLUE. And it also has four doors, and a back seat, and a trunk. But the "glue" part is what gets my attention.
Advantage for this comparison: ZL-1. You can only go so fast without going to jail. But HOW you do it is all important.