Great comparison by Joe Wilcox of iTMS and MSN Music.
http://www.microsoftmonitor.com/archives/003890.html
Me? I use neither. I used iTMS for awhile but realized that for just a little more $$$, I could have digital quality CDs, inserts, extras, etc. And I wouldn't have to worry about who had the best quality and who had the most flexible digital rights management.
In 3 years if someone comes out with a better format, I'm not stuck with 2004 technology. I just need to make sure I keep a computer around with a CDROM drive. I made that mistake with 5.25" floppy disks - up until a year ago, I had a box full of them and no drive to read them on. ;-)