Hi,
Piano sounds are very subjective , so my best recommendation is for you to try both keyboards out. One of my favourite piano sounds is still my trusty Yamaha EX5R and the stereo piano sound which really cuts through a mix when you are doing rock. In keyboard form the best piano sound I have heard remains my Korg Kronos and its acoustic SGX2 piano engine (streamed, unlooped samples with sympathetic string resonance modelling). But Pianoteq in VST form beats it on some things. It really depends on what you want.
I can't comment on the Roland, but I have both a Montage 7 and and the Prophet X, and I would say that if you want a very general all around board with very good expressive pianos and loads of other realistic sounds, then I think I would recommend the Montage. It's analog synth sounds are pretty good as well. Not quite in the same league as Yamaha's AN modelling in my EX5 and AN1x, but the Montage is very SY77 like to me in that the combination of the AWM and FM engines give you a huge amount of flexibility and can sound very organic - more than you would expect from a digital synth.
If you want a very deep analog synth that can also process sound samples, then it's the Prophet X.
Not sure if that helps or not.