I had the Kemper Stage and really had a couple of reliability issues. So much to german quality. So it returned the Stage. Now I have the FM3 and are really happy.
I like the sounds and especially the play feel on the FM3 much better. Kemper sounds good for Highgain for my taste, but i like the FM3 much more for the Fender, Dumble and other Overdrive and Clean sounds.
Regarding usability the fm3 is a totally different animal than the kemper. With the FM3 you get an editor which works perfectly. Softwaredeise the Kemper is years behind the others - and didnt' make much progress in the last years.