TOUGH call to make.
I think if you think like an analogue synth user and want to incorporate a piano into the mix, you go yammy. It's also got more immediate editing, because your modulation, preamps and 'verb are kick-inable from buttons. This DOES however mean it looks less like a piano, obviously.
Roland... the sounds are incredible. Dicking about with the hammer setting means that honky tonk/old piano sounds are incredibly achievable, and the clarity with the ambience setting is marvellous, especially the crisp studio touch and roomy hall echo.
Roland also looks better, hands down. However, the technician in me wants the CP1. Agonising times.