Well, the M50 has gone through a few variations that directly affected its bass response. That said, I think the two headphones are more comparable than not in terms of having a bit of additional, but clean bass.
The UE6000 has smoother but more laid-back mids and treble, yet it sounds like it has less of a U-shaped sound signature regardless. UE6000 sounds a bit wider and more natural/smooth overall.
UE6000 is more comfortable (over-ear vs pseudo over-ear) but not built as well.
It's been a while since I've used an M50, so my impressions could be highly flawed. But I am quite impressed with the UE6000. I wish it was a bit larger and had different shaped ear pads, as it can get uncomfortable for me. But the M50 was no better in terms of comfort.