C350 for sure, can't see the C250 or C63 AMG competing with the Q60 unless Infiniti plans to make some super high performing Q60 packing a V8 
It's tough to say why I would choose one over the other since info on the Q60 is very limited at the moment
It's tough to say why I would choose one over the other since info on the Q60 is very limited at the moment