My first thrust into Contax was a 139Q. I really enjoyed that camera and the 50mm f/1.4 lens I got with it. I wasn't big on the motor winder, which at first I thought I would be. My thumb still works fine after all these years, and I am more in control. 😛
But it was very accurate exposure wise. Most especially the TTL OTF(off the film) flash metering. That was astounding. I once took a photo inside a burned out building with the TLA 20 flash (almost classic black cat in a coal bin). I got good exposure indication in the viewfinder. I couldn't believe it until I got the prints. Perfect. If you like flash in areas where sufficient light will be reflected from all areas, that is a fantastic combination.
Many of the cameras from that era, Contax or Yashica will have leather problems. I don't know what Kyocera's specs were, or who they got those coverings from, but if you find one with good leather, it has probably been replaced. You can recover yourself, or just clean the rest of the outer leather(ette) off and use shoe die. That's what I did.
Unfortunately, the camera died. I never found out what was the cause, but at the time I didn't have the money to find out and get it fixed. However, as some time went by, I happened to look at what Yashica had to offer. I discovered that many of the Yashic FX line seemed very similar to the Contax line. I chose one. I liked it so much, I got two more before the prices started going up. I will now always have a body for my 50mm. As time goes by, I may sell some (too much) extra cameras I have to fund some Contax lenses I would like to have. Perhaps a 28mm and a 135mm.
The FX I got has the same OTF flash metering, accepts the TLA series flashes as well as the Yashica line of flashes. It has the same great normal metering. It also has a couple of other options for metering. Unfortunately, like the Contax, it is a paper weight without a battery. Knowing that when I got the 139Q, I also purchased an FX-3 at the same time. It is a very basic camera, with no frills, and no reliance on batteries except for metering.
Are there any Yashica and Contax cameras that have the exact same innards? I don't know for sure. There do seem to be some that have similar functions, so it may well be so. I don't care. I now have a Contax 167mt as well. I like it very much. But I still sometimes use one of my FX cameras because they are nice and then there is that pesky motor wind which can't be bypassed on the 167mt. 😀
I would say from my point of view, the Yashica FX line is fine. The 139Q and 157 never seemed as robust to me as the 167 or RTS. I never owned a 157, but they just didn't feel as robust. The 139Q did, but for didn't turn out to be.
Good luck in your choice.