Here's my thoughts:
I used them both for many years. When recently deciding to get back into medium format, I opted to go for the Contax. Here's my reasons:
(1) AF is slow on both cameras, but it's more accurate on the Contax (less hunting).
(2) Because my work doesn't often require high speed flash sync, the focal plane shutter of the Contax (1/4000th) is more useful to me than the top shutter speed of Rollei PQS lenses (1/1000th) for outdoor shooting. When I'm shooting in the studio with strobes, the Contax's 125th flash sync is fine with me.
(3) The used prices for most Rollei lenses are significantly higher than for Contax lenses. I put together my Contax system (body, battery grip, 80mm, 45mm, 120mm, and 140mm lenses) for less than $4000 from Keh.com. The fact that you can now buy the 120mm used for around $1000 is crazy. It's absolutely one of the best lenses I've ever used for any camera.
(4) For shooting portraits, I prefer the rendering of the Contax Zeiss lenses over either the Zeiss or Schneider lenses for Rollei. There's no question that some of the Rollei lenses -- especially the Schneiders -- are sharper than the Contax lenses (other than the 120mm) in the sense that they resolve more information. But resolution isn't the same as "look." I just prefer the look of the Contax lenses for shooting people.
(5) I can use my Hasselblad 110mm f2.0 FE lens on the Contax via an adapter.
(6) I owned both the Rollei and the Contax at the same time for a couple of years. Looking back thru my photos from that period, it's clear that I made noticeably better images with the Contax. That could be for a variety of reasons, not all of which are related to which is a "better' camera in terms of objective image quality (size, weight, "feel," etc).
All that said, I loved shooting with that big square waist level finder on the Rollei. (Contax does have a waist level finder, but it's hard to find now. And of course it's only really useful when shooting horizontally). And you certainly can't fault the Rollei glass; objectively, it's extremely good.
EDIT: with regard to repairs and service, the last I heard, this company does Contax repairs:
http://tocad.com/service.htmlOne disturbing part, however, is this quote from their website:
" Contax N Digital, RTS III, U4R, I4R, SL300R T* and TVS Digital cameras and 645 lenses are sent to Contax Japan for estimate, approval and repair. This process can take up to 90 days. An estimate is provided free of cost even in the event the unit is not approved for repair."
Obviously, waiting "up to 90 days" isn't an option for a working pro. In which case, I'd suggest just buying backups to the critical parts of the system because they're so inexpensive now.