I suspect the Grande Punto is probably the more ideal car for the US. I don't think people in the US really go ga-ga for supermini's anyway
The few Americans I talk to reguarly all seem to think the 500 would sell for novelty value to a few people but are MUCH more interested in the Grande Punto and larger cars in the FIAT range (and all seem stunned by the fuel economy and keep checking that they aren't ALL diesels, especially when I point out thats MPG using the UK gallon, which is 1.2x bigger than a US gallon)