i use all K&M but if i was you id take a look at the PMA tools...i like how the mandrel moves to adjust the cut depth on the neck instead of the blade...the 21st century is a nice tool as well because the the mandrel moves up and down with a micro meter adjustment for the cut depth into the shoulder...the forster has a wider blade which is supposed to make a cleaner cut but ive had no issues with the thinner blade in my K&M.

make sure you buy as a kit with the expander,blade,mandrel and the power adapter...all the tools are nice just pick one with the features you like...and just so you know the bodies of these tools are small...the K&M has a ergo adaptor and if you get the K&M id suggest you get it...the PMA tool is a little bigger.