There were two series of chrome lenses produced. The first had a top speed of 1/400 of a second, the latter improved the shutter speed to 1/500 of a second. These had different shutters and shutter cocking mechanisms than the later black lenses.

The biggest differences in the two series are optical quality, shutter type and production run. The black lenses are better optically and more consistent optically than the chrome lenses. You can get some very good chrome lenses but you can get some dogs too. The black lenses were also manufactured in higher volume so finding replacements or replacement parts is much easier. There are still lots of chrome lenses out there but not as many as the black. The black lenses also had an improved shutter mechanism. More reliable over time in terms of maintaining speeds and better constructed to last longer.

Both the chrome and black lenses could have been coated but not all were. I do believe more of the black lenses were coated than the chrome though. The black lenses are also newer so you're buying a piece of equipment that's not got as many years of use on it which could be beneficial. There were also more focal lengths of the black lenses produced giving the photographer greater flexibility.

Here are a couple links to some very good resources on the Mamiya TLR system.
http://www.btinternet.com/~g.a.patterson/mfaq/m_faq-contents.html
http://www.williamsphotographic.com/ocr/mf/mamtlr.html

As an aside (and I don't know if this is permitted here) but I'm looking to get rid of my TLR system so if you're looking for lenses or another body, send me a PM and I can let you know what I've got.

Thanks, yes let me know ... cfstokes@hotmail.com