The HS-322S is no longer listed in the
Hitec catalog, only the 322HD. The only difference between the two is that the 322s uses white nylon gears, while the 322HD uses a harder (but more brittle) plastic called "Karbonite", which resists wear much better but is more susceptible to breaking under shock loads.
As far as I can tell, the 322HD replaced the standard 322S.