There has been a lot of discussions concerning the fit of these shoes in the forum for some time now. It's been apparent, you need to try a shoe before purchasing. For my, the Classics and the ICONs fit exactly the same (12, M) but that isn't the case for all of the member here that have come from Classics to ICONs.
Mike and Chris have a lot of great information concerning fit in the ICONs compared to the other two options, but your best bet is to run to an authorized dealer and give them a try. Everyone's foot is different and the fit may be different as well as you have experience with the Classics to Synr-G move.