Welcome to the forum, I've always loved Mustangs and since I was a kid have wanted one, one day I probably will have one some day.
That being said I also have always wanted a Z just as much (my first car was a Datsun).
I there are two main reasons I chose my Z, and it has nothing to do with which is faster, because they are both blazing lay fast cars.
For me it was about styling and handling, and also about how many are out thre on the road. The other day I had a nice Mustang pull up next to me and I looked over and thought it was a pretty nice car, but then I looked in my rear view and saw another one, and then another one in the oncoming lanes across the intersection. Maybe that's a testament to the cars popularity, but I've always enjoyed being sort of in the minority, but knowing what's best.
I've always been that way (i.e. I've been a Mac computer user since 1990, back when they were at their lowest, but I always felt they, we're better, sleeker and a better design.
Again welcome to the Z