The Artisan is a very good mixer for the home baker but the 600 (which I use) is much stronger for heavy mixing chores.
Williams Sonoma list of differences:
Horsepower, control panel (I believe this is to maintain the speed when mixing varying densities), upgraded included accessories, and planetary mixing action (the rotational pattern).
To answer an earlier question, the Artisan series (and all flip top series, AFAIK) use “nylatron” gears which is a mixture of nylon and kevlar fibers. In use since 1998.