I've only handled and shot FAs with a 6" barrel. I've got a 6" Premier .454 with micarta grips plus a 3lb trigger job and my friend has got otherwise similar .44, but with wood grips.
We've mostly shot the guns at 25 meters, but I think that the 6" barrel offers a good compromise concerning handiness/sight radius. The 7,5" barrel will add some fps, but my friend's .44 FA chronoed decent velocities too.
I've handled (also a 6" model 83") one with the Express sights and I don't think that I'd be getting those. For me they seem to make it more difficult to see when the front sight is on the level with the rear sight. But then of course I'm so accustomed to regular notch and post type...
While I'm still happy with my choice to go with the 6" barrel, now I want to buy a 4,75" round butt, a 7,5" octacon and so on. Others may have warned you already, but be prepared that you'll want more!