I'd have to side with Logitech on this one.
1) MS mice have a cheap feel to *me. Logitech's do not. (YMMV, go for what you like. I don't like buttons that feel "loose")
2) The MX series seems to track better (we use MS mice at work, I ONLY use logitech at home)
3) Um.....isn't MS a software company (LOL, reaching on this one

)
To be honest, I just bought a MX700 a week or so ago. I love it. I did, however, consider other mice (IBM, MS, Kennsington, etc), and still feel that the logitechs not only feel better in the hand, but that they are built a bit better. Having said this, I've always had a pref for logitech mice. Take it for what it's worth.