I also have a Miami Classic II. Galco makes at least four main shoulder holsters, the Classic I, Classic II, Jackass, and the Vertical Holster System. The Jackass and Classic I are fairly similar, and I can't at first glance distinguish a difference.. Most of them are basically the same but then vary on the angle the magazine pouches or holsters themselves are worn... i.e. vertical or horizontal.
Galco is probably one of the main production holster makes for shoulder rigs.. First, because they're even more expensive than a good IWB wiht more leather and what not, and because they got a lot more than their foot in the door on the free press when their rigs were on the Miami Vice TV show.
A shoulder holster is nice if you don't want the weight on your belt, but it you're a larger guy, the reach can be tedious. Additionally, for most of them, you'll be continuously pointing your barrel at anyone standing behind you. Not something that prevents me from wearing my rig from time to time, but it's definitely something I keep in mind when I do.