I have driven both at the track, cars were owned by 2 of my students.
Your choice of either Fiat or Mini indicates that your looking at a quirky car. The Cooper S is a good car, NOT a great car, it has some major bad points, expensive, heavy, and very little driver feed back. The Fiat is top of the quirky list, cute, expensive and also underpowered, and has the magic Abarth name, except in my opinion it isn't radical enough to wear the name.
As a Smart owner you already have the ultimate quirky car, buy the Fiesta ST