This is a great thread... although I guess my post is about to go off it slightly!
The road test comparing the performance of the cars is pointless from my perspective. We did not buy our Mini based on performance, and were not considering the 500 on performance either (but for what it's worth, the Cooper leaves all of the 500s behind anyway!). What we were comparing on was value, quality and grin factor!!
My wife and I ended up buying a 2006 Cooper for her to use, but not before testing the Fiat 500. We loved the style of the 500 (OK it's girly as posted earlier, but my wife loved it!). We loved the modern blue tooth connectivity and USB port for direct music input, and also the multi function steering wheel, all of which leaves the Mini behind. But the 500 ride was terrible (choppy and over hard), the seating position was bad (the drivers seat tilts rather than being height adjustable, and there is the classic Italian long arms short legs positioning of the steering wheel and pedals, which given the lack of reach adjustment on the wheel makes it impossible to get properly comfortable. Then there is the rear headroom. You need to be less than 5'9"(ish) or only take kids in the back. The rear leg room is OK, but no better than the Mini in practical terms. The mini looks like it has less leg room, but it's more that the front seats slide further back and the rear seat is full sized, rather than having a narrow bench as in the 500. In comparison the Mini feels spacious due to the light interior and good headroom.
Then there is the build quality. The 500 dash and interior trim was clearly marked during the installation (a feature that showed up on all three cars we drove), the rear view mirror felt very flimsy (so I was not so surprised that an earlier post in this thread said it came off during a test drive!), and the interior plastics on the 500 are not up to Mini standards by a long way.
And the final nail in the 500 coffin for us was the 500 servicing costs. It needs servicing every year... £120 first year, £240 second year and £340 third year, then repeat the cycle until you run out of money!

Compare. that to the £150 for 5 years Mini TLC pack plus £350 for the next 3 years and you will see that the 500 costs £1760 over 8 years compared to £500 for the Mini! now the Mini is cheaper than the 500 and still far higher quality! My wife kept her last car for 7 years, and really loves the Mini, so plans to keep it at least that long so these figures matter to us.
And if we needed any more convincing, every time we drove the Mini we loved it more, and every time we drove the 500 we liked it less!!
We are now several weeks into owning the Mini and LOVE it !!

I still like the style of the 500, and I'm sure it will make a good city car, but in my world it is no competitor to the Mini.