The GX or SX model?
The GX is the current updated version, the SX being the redundant model although some are still around in the cut price or second hand market mainly.
The good thing about the P-155 is that it will have its own speakers which means that it will sound as it was meant to. The Roland is a stage piano without onboard speakers which therefore needs external amplification.
The Roland RD-700GX represents the best Roland has to offer in a stage piano and will have a vairety of quality sounds. The P-155 is not the in the same league and falls within Yamaha's mid-range stage piano range. Unlike the RD, the P-155 would not be seen on stage as a gigging instrument. Having said that, I still think the Yamaha piano sound has more merit but the instrument would be nowhere near as versatile.
I suspect that the tiny speakers they have installed on the keyboard isn't really doing it much justice either. The main piano voice sounded as if there was too much mid-range and, of course, on the P range, there is no built in equaliser or external equaliser sliders!