I have the M3 & the Motif XS, just a little over a year now. They are both excellent workstation, with great feature sets. They do sound different. The XS has a more rock-&-roll sound with excellent real sounding instruments. The M3 is more electronic, synthesizer sounding. If I could only have one, I'd buy the one that complemented my other hardware.
I got a Roland Fantom -- get the M3
I am in a Rock Band -- get the XS
I Love Dance Music -- get the M3
TI Virus Snow, Radias, Nord Lead 2X, Motif XS6, M3, Fantom X7/G7, Karma, Wavestation, M1, D50, JP8000, microX, microKorg, DX200, AN200