John,
I don't have any wisdom, but I can give you my opinion (opinions are cheap!) I really like the autofocus of the AX. I find it more convenient than the focus indicator and DFI in the RX. So if the reason to choose is "focus help", I'd choose the AX.
Regarding handling, the AX is significantly bulkier, and the RX "feels" more solid and higher quality (the all-metal construction, maybe?) so I think it is important to hold both and, ideally, to try both. (I am sure both are similar in quality, regardless of the "feel").
The AX is more complex, so I guess more things might go wrong. It may be more fragile, I have no way of knowing.
I have been happy with the AX autofocus, but this may be because I almost never photograph anything that is moving...If you used the AX and did not like it, that should be the end of the discussion.
Don,
Regarding replacements for the original RTS, my experience is with 167Mt, RX and AX. The 167Mt is a very good camera, but I never liked the fact some controls are buttons instead of dials. If you shoot mainly aperture priority, it is not much of an issue. The camera has everything I wanted at the time: spot metering, exposure lock and autobracketing.
The RX is very similar, with a darker viewfinder but more and better information on it (plus the focus and DFI indicators). The AX is somewhat of a monster size-wise, and the spot metering lever is on a different location than on the 167Mt and RX. The autofocus works nicely, and if you like macrophotography, it has the added bonus of the "macro capability". I have never even seen the 137, 139, ST or Aria.
Best regards,
Juan