I got an old - probably 30 yrs. - Reloader Special, and it was always a smaller frame press with less meat in the linkage. Best way I know would be call RCBS and ask them what are the differences. They are great help.
Ive got a Reloader 3 I bought off E-bay as a backup. NO WAY is it going to replace the RockChucker...period..! The reloader series are good presses but they just arent the RockChuckers is all. If buying NEW go the extra $$ and get the Chucker or you can always check E-bay out and LOOK CLOSE at the pics and pick up a good used one....after all...ya ant hurt one.
The RS series were less costly, smaller, lighter presses and had a simple toggle linkage which reduces the leverage. At least one had a cast alum alloy body. They are fine but work best for handgun and small rifle cartridges.