I would stick to the Korg operating system too. As you say, Yamaha's samples of natural instruments are so often regarded as more realistic than Korg, but I made the opposite experience comparing many Korg M3XP sounds to Yamaha Motif XS sounds. There was a giant leap from M3 to M3XP, and especially the brass&woodwinds expansion had breathtakingly realistic samples I didn't have in the Motif XS presets (or MoX, exactly same 355MB waveROM). So, if the PA 3X allows to have expansions of the quality of M3XP, I don't think you need a Tyros for the sounds.