Uhm you can go onto the BMW official web site and look at the spec. diff. between the two. Basically the M version is a High Performance sport edition, upgraded enigine, suspension, sportier interior w/ more options. Examples of others like this Chevy's SS, FORD SVT, Dodge SRT, Toyota TRD, Honda Si, MazdaSpeed, Nissan Nismo, Mercedes AMG, Audi "S"..and so on and so forth.
If I had an option, I'd have an M.