The classic is a stand alone light system, and attaches with a proprietary slide stop and two set screws. It will mar the finish on the frame in short order, but it's a good light. Compared to the X200A and B, it's very heavy, and it will not pulloff the gun in a hurry. It requires tools for installation and removal, and you cannot use it with the Dawson rail.
The advantage to the Dawson rail is that it is unobtrusive, you can use almost any regular 1911 holster instead of a holster that is specific to a railed gun, and it's well made. The disadvantages are it is a little pricey, requires drilling and tapping of the frame (it permanently modifies the gun), and it requires gunsmith installation if you don't have a drill press and taps.
Also, the adapter that fits the Surefire X200A/B will not fit the other Surefire lights like the Nitrolon. You would need a separate adapter.