I wouldnt follow these answers too closely...
It depends entierly what you want from the synth, Korg is much better @ pads and synth stuff (+ X50 is way more of a synth than yammy).
Yamaha has better acustic sounds (more layers + "mega voices" + better samples). Anyway X50 is way less of a small brother of triton than MM is small brother of Motif XS. X50 acualy has more sample rom than triton, but not as many IFX (still more than MM though, I think).
So yeah, depends what you are doing, for acustics go yamaha, though sample librarys are much better for that stuff anyway and MM is more of a "toy" in synth sounds, it is better built than X50 though. X50 has much more to edit I do rather take it but if you just want piano sound than the choice is clear.
So conclusion: X50 = better synth/pad/arp more layering better orchestra sounds
MM= Better keyboard and acustic sounds, much better keyboard and built too.
They are both kinda crappy though, but the X50 does have some very usable pads and IT IS a triton if you have a FX rack.
Also Juno-G is the best one engine wise and has nice amount of expansion boards you can add, which are probably better than koeg or yammy, though it is easily the most expensive.
I wouldnt take any of these really, cause in the end you would be disappointed (well X50 does have some good pads...)