Chrome lenses use an older Seikosha shutter. As gtyler5 mentioned, parts are no longer available new (though so many of the these shutters were used, "donor" shutters can sometimes be found. The black lenses use the later Seiko shutter.
Some of the later black lenses have a blue dot on the shutter cocking knob. I've heard differing stories about these "blue dot" lenses. One says basically it's the latest version of the lenses, the other says it's a better version of the lenses.
I wouldn't shy away from an appropriately priced chrome shutter lens, but the black shutter lens would be preferred.