You won't get a European car; they're all described as "premium". You'll either get a domestic (Chevrolet Impala/Dodge Charger/Ford Taurus) or a far eastern (Toyota Avalon/Nissan Maxima/something Korean). And don't hold out hope for a V8 - most now have 4 cylinder engines, with optional V6s (which you most likely won't get, as it's a rental car and therefore will be the cheapest spec. they can buy). You might get lucky and get a V6 if the manufacturer had too many in inventory and wanted rid.
I'd say if you get the choice, go for the Impala; they're supposed to be quite good. I had a Malibu (mid-size) on rental last time I was in the states and it was better then I was expecting - not Audi/Mercedes build quality, but pretty good for an American car. They are built to a price, and generally it shows.
Good luck!
Cheers,
Plug