Test drove an '11 Mustang GT before getting my M3. It had the California Special package and the 3.55 rear end. It definitely felt faster than my old modded 335i and had a nice exhaust note, but as I was exploring my way around the interior, I kind of took for granted how well BMW's are put together. In the Mustang, there were interior pieces that wiggled around a bit that weren't supposed to (like the center console cover). The clutch pedal was annoyingly soft. It had the Shaker 500 stereo (not the optional 1000), and it sounded kinda crappy TBH. However, the options and packages I desired weren't on the car (Nav, heated sport seats, backup camera, etc), and I was hopeful they would change my mind about the car for the better, so I put a deposit down and created an order for one which would have been ready by the end of the year.
In the mean time, I was still looking at M3's because deep down I wanted to get back into a BMW (but didn't want the lovely fuel pump drama of the N54). I test drove an e92 I was interested in, and the interior was soooo much better. The ergonomics and quality of materials far surpassed the Mustang's. The drive... the sound.... Everything felt right. So I cancelled the Mustang order and picked up the M. Couldn't be happier