Mark VII's and Mark XIX's both have adjustable triggers in them. For either, you can get longer barrels (10"). Like fuatos said, for the Mark XIX, you can get the other calibers as well as conversions (or you can buy the whole damn set, but with something like 6 barrels, I imagine it is damned expensive). The Mark XIX does have a scope rail as part of the barrel, complete with the crosscuts for weaver style mounts. Also, since the Mark XIX is based on the Mark VII, parts for the Mark VII will work in the Mark XIX.
I have shot the Mark XIX .50AE, and it is a fine gun. I just didn't see the need, so I bought a Mark VII .44Mag. Love to shoot the gun and you always get some looks when you pony up to the line and let the thunder loose!
Either way you go, you will be getting a hell of a weapon and will enjoy it a bunch.
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WWoodworth