Combis (aka Performances on Yamaha) is a place where you combine, split and layer several programs (sounds) at once.
For example - you put a pad and a bass on the bottom half of the keyboard, and a piano + strings on the upper part.
Each program/sound in a combi/performance will take 1 spot. The M50 can use
up to 16 programs at once - while the MO can use only 4.
Depending on how complex you want your combis - this can be a major issue.
And about the pianos... like I said - it's a very personal opinion for every player, and
you will be best off if you just tried both keyboards out. My opinion is that the M50
piano is a major improvement over the Triton series, but still it comes nowhere near the ones on Yamaha keyboards.
One more thing I noticed is that the new pianos from the ESD synthesis (the one found on the M50 and the M3) do not cut out in the mix at all.
If you play live in a band this could be a concern for you. I tried the factory pianos
on my M3 in 4 different bands on 4 different amps and every time i got the same result - it didn't cut through the mix at all - no matter how much i turned up the volume.
All i would get was massive noise instead of the clear piano sound. That's why i stick to my CP33 when it comes to Piano sounds.
But if you'll be playing at home - don't even take this as a concern
Pads on Yamaha keyboards are not "creamy" or lush as much as the ones on Korg.
So they won't blend in the background so smoothly as the ones on the M50 would.
So if you want deep, fat, creamy pads/motion synths... you will need to dig into programing on the yamaha.
Same goes for every acoustic sound on Korg