I have an extensive history with Camaros and Mustangs going back to 1975. Here's the list (all manuals):
1974 Camaro Z28
1980 Camaro Z28
1984 Camaro Z28 (L69 H.O. option)
1987 Mustang GT
1995 Mustang GT
2005 Mustang GT
I was strictly a GM guy until the late '80s when a co-worker said his 5.0 Capri could beat my '84 Z28. I just laughed it off. One night after work we met unplanned, side-by-side on the highway, and sure enough his 5.0 pulled away from my 190 horsepower L69. Instantly changed my allegiance from GM.
Ordered the S197 2005 Mustang when it was the hot redesigned pony car. Camaro had been discontinued since 2002. I have nothing but praise for that Mustang. I got it with the Screaming Yellow paint, and it was like being a celebrity the whole 10 years I owned it. I learned about track days with that car, and did one HPDE event each year from 2007 until I sold it in 2015. The car never went back to the dealer once, and never had a single problem the entire time I owned it. Rock solid.
To answer the question from the OP - Why DIDN'T I buy a Mustang GT? Basically a test drive in a 2SS sealed the deal. It was love at first sight with the exhaust note and racecar-like feel. I did test drive a Mustang GT PP1 as well, and I liked that car too. The Mustang ergonomics were better, but I was already in love with the Camaro and it was a foregone conclusion.