I guess I get to be the lucky one who is the first actual owner of an X50 to post to this zombie thread in the last couple days. The X50 is a whole lot more of a Triton than the MM6 is of a Motif. The MM6 is a PSR in Motif clothing. The X50 is a real Triton. The X50 is worth buying over the MM6 merely for the VST editor.
I've been contemplating selling my X50. My analogy: it's kinda like bad chinese food. It tastes good for a while then it has a crappy aftertaste.
I guess I'm not a Korg fanboy, the only other thing I've owned from Korg was a Poly 800 back in the dark ages. Everything on the X50 sounds artificial and processed. The first year I had it I really liked it, but lately I use it less and less, and the sound is almost irritating to me. I think it's the reverb.
For most people, it would be the better choice by far, no doubt. I think I've come to realize that I'm more of an emulative guy when it comes to synths, it's my classical geekdom upbringing. I want an orchestra in a box. Still, the X50 is an amazing unit for a great price, I just think I am happier with a more acoustic soundset.
My next step is to buy a better computer and get some serious samples. The East/West Goliath set looks pretty interesting.