The FR series is considered a residential grade engine, the FS is considered a commercial grade engine, the FX is the top of the Kawasaki engine line and has the two stage canister type air filter system. You can go to the Kawasaki small engine website and read what they say about each engine line.
I would say that the $500 difference is primarily in the engine.
I recommend the Hustler as I've had several and they are fine machines!
Mad Mackie in CT