The OS .46 FX has ball bearings supporting the crankshaft; the .46 LA has bushings. Ball bearings mean less internal friction, more power and higher cost. A .40 FX puts out more power than a .46 LA. The FX also has a remote needle valve which keeps your fingers further from the prop when making adjustments. Neither of these is ringed, meaning that neither has piston rings.
OS engines are higher priced than many others out there. They are high quality engines, but other, less expensive engines work well too. You will find fans here of Super Tigre, Magnum, Thunder Tiger and others which cost less than OS.
Whatever engine you buy, pay attention to the fuel and break-in procedure recommendations of the manufacturer. They will make a big difference in performance and longevity.