From my experience with running 175W XM bulbs and 175W Ushio bulbs, the XMs appeared crisp white, while the Ushios were white with a hint of yellow. My experience seems to be consistent with what I've read on various message boards.
I just switched from the ushio10k to the xm 10k and like the xm alot, seems to me to be a crisper white after the burn in, I had used the ushio for years and just made the switch because I had heard alot about the xm plus the price is a big plus.
Just got my new XM 250 watt bulbs and I like them very much. I just wish I had a PAR meter to see what they are really putting out.
I agree with what RobChuck says about the difference between the two bulbs are. There is a bit more yellow to the Ushio's and the XM's are putting out a very nice crisper white.