I have never used a Contax G2, although I saw a test somewhere which tested one of the lenses (forget which) against a range of other cameras. On that test it did quite a lot better than everything else. That's just one test though, and I'd suggest you try to hunt it down. Theoretically, I understand that it's easier to make a high quality lens for a rangefinder camera as you have more flexibility with how far the rear element is placed from the film/sensor. With SLRs, it needs to clear the mirror, so you can't have it too close.

However, I would say that even if there is a difference, it'll be non-existent compared to the difference caused by camera shake and other factors.

If I was spending $1500 on a 35mm camera, I'd probably get a Zeiss Ikon, if you're after other suggestions.

If you like SLRs though, stick with that, as you get so much more bang for buck.

Printing 20" across I'd say it's up to your own standards whether it's possible to get good results. I'd likely be happy with it, others would not be.

Also, here is a far out suggestion for you. You like medium format, you want a camera you can take everywhere, how about a Fujifilm GF670, Plaubel Makina, or Mamiya 6 or 7? All very portable medium format hand held cameras.