All 212 Ortofons are 9" tonearms (212 mm Pivot to spindle).
I'll take the AS 212 over the RS version - the RS is a very good tonearm, yet the AS was measured with some of the lowest friction numbers ever, and is a truly amazing tonearm. Surprisingly enough, it also costs less.
IMHO - the SA-750D falls somewhere in between.
BTW, the tonearm's "role" is to enable the cartridge to sound its best, and all are more than good for the task, so with each, your turntable will sound as good as the cartridge mounted - as long as its a match for the tonearm, and set up right.