We looked and drove both the Fiat 500 and the Mini Cooper. We bought the Cooper S as it felt hands-down faster, tighter and better built than the Fiat, at least to us. No mechanical issues whatsoever after 14,000 miles.
The only negative I have with the car is the front seats are brutal. I'm 6'-5" 210 lbs and have more than enough head and leg room (even with the sunroof), but the seats are just too narrow. I'm not exactly a fat ass but can't wait to get out after about 40 miles. They're awful.
Other than that, it's a wicked-fun car. Fortunately, it's my wifes daily driver or I'd be getting into trouble with it.