I enjoy using Leica cameras as well but I sense that some find their Leica cameras easier to work with because they are more comfortable with them. This is certainly understandable as there are a lot more Leica cameras out there to work with than Contax cameras. However, my experience with these cameras is a bit different.
My Contax II is a wonderful daily user and I have been shooting it almost continuously since I got the first one. I own a Leica IIIc, a Tower Type 3, and an M6 but IMHO, my Contax II is easier to work with than all of the Leicas I own. Rather than finding my Contax to be an interesting, but quirky, camera that is fun to play around with every now and then, it is actually more of a day-to-day user for me, and my Leicas are the interesting, but quirky, little cameras that are fun to play with now and then.
Leica LTM cameras are lighter, smaller, and less bulky than Contax cameras. There is not enough difference between the M style and the Contax as far as weight and size to comment on. It is easier to slide an LTM camera with the Elmar 50/3.5 in your pocket, or purse, than the Contax. However, I personally find the Contax easier to grasp and handle than the smaller Leicas. I think that is more a reflection on my long experience with Minolta SRT and Pentax K cameras than any superiority between Leica or Zeiss Ikon design.
I much prefer the single viewfinder/rangefinder combination of the Contax is easier to work with than the LTM camera separate viewfinder/rangefinder arrangement and about the same as the M. Interestingly I do not find the combined film advance/shutter speed dial a great advantage for the Contax, mostly because with my model it is harder to see the actual shutter speed numbers in real life use.
Film loading is WAYY easier with the Contax. I hear a lot of people complain that the take up spool falls out. My experience is totally the opposite. I actually pull the take up spool out to load film. I push the tongue of the film into the slot in the take up spool with both the spool and the cassette right out where I can see both. Once the film perforation has been hooked on the spool, I twist the take up spool about a half a wind or a little more and then pull the film tongue out as I am inserting the film cassette and take up spool into the camera. Once they each are in their respective locations, I lay the film back in place, slide it up, and lock it down. I then wind the film on one frame, snap the shutter, wind it forward once more, and I am ready to take pictures. It has never failed. I suppose that if I were under enemy fire on the front line like Robert Capa then I might miss once in a while, but I'm not. I can load film faster in my LX with the magic fingers, but not by much. I don't even want to discuss the fiddly little Leica!!
I could go on, but the truth is that anyone can get used to, and like, the camera they use the most. For example, I really like my Minolta SRT 102...and I use it a lot. Likewise I really like my Contax II...and I use it a lot. Now, which came first, the chicken or the egg. Did I like these cameras first, and therefore I began to use them a lot? Or do I like them because I use them a lot? It is probably a bit of both but familiarity born of regular use does count for a lot.
Obviously the Contax IIIa you are considering came with a meter and neither of my Contax II cameras have that feature. The camera light meter may or may not work, and if it does work it may not be reliable. Actually, the Zeiss Ikon meter cells have a pretty good track record for holding up, and the one you are looking at may work. However, I would recommend that you consider a separate meter and not try to use the built in meter, even if it does work. Now days hand-held light meters are very reliable, usually quite a bit more reliable than any of the older light meters found in used cameras. But, if all you shoot is negative film, and your camera's light meter is fairly linear, even if it is off a bit it won't matter too much as long as it is off equally from one setting to the next.
And yes, the Zeiss Ikon lenses are truly stunning. I first started shooting these cameras in order to get my hands on those Carl Zeiss Sonnar and Planar lenses, and I do not regret it one bit. In fact, you could say I am really hooked. As a matter of fact I think that my 50/1.5 Sonnar and 85/2 Planar are just about perfect. If I could locate a 180/2.8 telephoto I would be content...I think!
Anyway, buy the camera and enjoy it. You may not use it like I do my own, but you will still get a great education in a wonderful camera from our past.