if i got an hrc i would only get the push version. no tranny to worry about that way.
i don't use a 21" that much so i'll just get the hrx2174vka version with the slip belt arrangemet. only thing that can go bad tranny wise on those is a belt. easy to replace and low cost to buy as well.
i have a craftsman 21" with honda engine now that uses a slip belt arrangement for the self propelled part. i've had it over 10 years and still using the original belt with no problems.