In short, M1's offer more "precise" movement (1/4 MOA as compared to 1 MOA) and IME, are just plain more user friendly. The argument can also be made that they're less likely to to get bumped in the field, but I couldn't exactly prove it.
http://www.opticstalk.com/forum_posts.asp?TID=15845
Do a Google search of "Leupold M1 vs M3" and you'll find all kinds of reading.
And as a side note, I've preferred M1's in the past to any other Leupold offering, but any future purchases will be fitted with CDS dials.
