The XP-50 and the XP-60 have both great sounds and they are OK if you are going to use it just to play on it.
The XP-60 has more functions and better control than the XP-50.
But if you are going to use the sequencer (on the XP-50 or XP-60) to make and record songs with several instruments on, youŽll have big trouble sooner or later. Sometimes the drums canŽt keep in time and voice polyphony doesnŽt come up to the 64-voices as it should. On the contrary, far from that. There are some more defects too.
If you intend to buy one of those synths you should really try it before, specially the sequencer.
Good luck!!!