Gandy's site, for which Lee provided a link, is a good source of information on the different Leica models and variations in markings and such. Some of the variations in markings have additional appeal to some folks - generally those who collect, rather than use the cameras. The Wetzler marking, for example, may have additional appeal, and thus carry some premium in price. Except for some of the "special editions", as I recall, the body materials are the same, unlike the lenses. Chrome Leica M lenses have brass barrels, and are substantially heavier (and usually more expensive) than the black annodized alloy models.

With bodies, the chrome vs. black issue is really a matter of aesthetics and personal preference, perhaps with a touch of practicality thrown in. For example, I have two otherwise identical M6TTL bodies, one chrome and one black. I use the black body for B&W film, and the chrome body for color. That way, it's easy to grab the one I want from the bag.

For an M6 that you plan on using, I'd pay more attention to features and general condition, rather than production location. With earlier production models, it's also a good idea to find out if the camera has had recent CLA servicing. I'd also give strong consideration to the reliability of the seller. The Leica market seems to attract more than its share of dubious sellers who often don't deliver what they're advertising.