Contax 645s were never very popular here on the West Coast (CA) so I have no point of comparison on the ergonomics. That is a worthwhile consideration as I'm not as likely to carry a camera that's frustrating or unpleasant to use. I can tell you that the Contaxes I've seen on eBay have tended to hold more of their original, as do the lenses. Pentax gear is dirt cheap by comparison and I find the image quality leaves little to be desired. (I'm a nit-picking pro, incidentally).
If I was in Germany where they might be found in numbers, I'd add the Rollei Hy6 to the list of cameras to look at, myself.