I bought the Fiesta because it was cheap, and I heard a lot of good things.
In all honesty, it's an alright car. I don't love it, but it can be fun if you got someplace to turn it. Mustangs are just fast out of the box and a tune goes a long way. The Fiesta really needs a turbo upgrade if you spend a lot of time on the highway like I do, it's just too under powered otherwise. My V6 Sienna would probably outrun my Fiesta as it sits.
I mean, I get why people buy Mustangs. I was just interested in the correlation between the two. (I'd kill for a GT350R.)
I don't do straight lines on my FiST, or rather I only do them, so as to get to the twisties. I don't commute or around town, either. No drag strip, roll racing, autox or track, either. Just twisty madness, along with the ride to and from.
The Mustang wouldn't be the right car on some of the 1.5 lane roads that I frequent. That said, I wholly get its appeal.