I have 139Q, 159MM, Aria and RTSII in more or less active use so I thought of sharing by experiences on those:
-All are well designed for aperture priority shooting. Exp compensation is on right hand and shutter speed is on left behind a lock. I like this design since I am 99% on aperture priority but for manual shooting the "traditional arrangement for controls" would probably be more convenient.
-Having metering button separately (RTS, 139) is comfortable compared to the one integrated with shutter (159, Aria) since the shutter touch is quite light, actually "feather light" on RTS. Having it this way reduces accidental exposures. Also having the AE lock in the same place is convenient. AE lock on 159/Aria is around ths shutter so it is also well placed.
-All support at least ASA3200, only Aria supports DX (can be overridden)
-Center weighted works really well - thus I would not put much weight on matrix metering. I seem to be getting constant results with center weighted but sometimes get small underexposure when using Arias matrix metering. Aria has spot metering as well. Aria and 159 have program and shutter priority modes (which I never use) which require MM-type lenses.
-Aria and 159 have the fastest shutter 1/4000, RTS - 1/2000 and 139 - 1/1000. On sync speeds the winner is 159 (1/250), Aria has 1/125, 139Q 1/100 and RTS only 1/60. Thus I use mainly 159 when having a shoot with flashes. All support TTL with Contax flashes.
-RTSII is by far best damped, 159/Aria are pretty similar and 139 is least damped, but not bad either. If I am sloppy on technique I think RTS gives me one stop advantage on shake compared to 139. When I am concentrating on shooting properly all are good.
-I think Aria is the most quiet, RTSII and 159 pretty similar (RTS maybe having a small advantage) and 139 gives definitely the loudest "clonk".
-All have pretty good viewfinders and all provide necessary info for me (speed, aperture, compensation active/not). Aria shows the most info, even showing live the exposure difference compared to center weighted when on matrix metering. I think RTS is easiest to snap on focus evenwhile I have only microprism whilst all other have split-image in addition to microprism.
-RTS has separate mirror-up function as well as one mechanical shutter speed (1/50).
-139 and 159 use easily available and cheap LR/SR44 batteries. 139 runs pretty long on a set of fresh batteries. Aria uses CR2s and uses the most juice. RTS runs on 4LR44 so I think one could use a stack of LR44s as well. 139 does not have a separate power switch which means accidental exposures when putting the camera into bag with shutter cocked.
-RTSII has by far the most solid build and is the biggest and heaviest. 139 is pretty solid and small, 159 having bit more weight. Aria is the most flimsy/plasticky of them all.
-Aria has motorised advance, others are manual. Film advance is smooth on all Contaxes.
-139 has the least convenient (but not bad) take-up spool, Aria is naturally fully automatic and 159 and RTS are very handy to load. RTS has the best implementation of rewind crank.
If buying, all but Aria can suffer on the mirror slip, which is not that hard to fix. All can suffer on contaminated shutter magnets which means shutter not releasing or sometimes not releasing. 139s have a mechanism between AE-lock and advance lever so that when advance lever is pressed in it also disengages the AE-lock in order not to drain the batteries. This has been broken in many 139s I've seen. On one of my 139s it jammed the advance mechanism, but that was easy to fix by taking the top off and removing the broken piece. Naturally then that linkage no longer works, but otherwise camera became back to fully functional state. Most of the issues I've had with my 159 (mirror slip, contaminated magnet, mirror shifting and causing focusing error). Electronics have been OK on my cameras, but as someone said they are so old already that if something on electronics would have broken it would've happened already.
All and all, when the main objective is to take pictures I typically pick up the RTSII. If I am doing a "studio type" flash shoot I pick up 159MM and the all-rounder and travel camera is 139Q. For some reason Aria gets very little use. If I had to pick up only one I would choose 139Q - for me they have been robust, small and contain all that is needed but nothing that is not essential. And they are cheap to buy and use.