My vote is the butt cuff. I don't like having the shells right at the center of balance on the shotgun, where I pick it up. It also makes it bigger around at that point, and seems more likely to lose its shells.
You have to practice a little with the shells on the buttcuff, butt it's easy with a little orientation. You have to decide for yourself whether to go rims-up or rims down. I like rims down, so that I can pull the shells out from the bottem with the shotgun shouldered. This means that you'll be replacing it in a few years, however, because you want to make certain that the shells don't fall out, and the elastic gets tired after a while.
Also, with a nylon buttcuff, you need to burn a hole through the bottem of it with a hot nail so that the buttstock sling swivel can just barely push through, as a tight fit. You don't have a sling swivel on your buttstock, you say? Then get one! The swivel stud keeps the cuff from riding up the stock toward the pistolgrip (as it is wont to do), and a sling makes your shotgun far more useable, tactically. (For a shotgun, it needn't be a proper super-adjustable sniper sling; a cheap web carrying strap will actually serve pretty well for carry and hasty sling.)
My cuff came from Uncle Mike's and is called a SideKick. My dad's was some no-name off-brand, and does the exact same thing, but cheaper. (I doubt there's much of a quality difference, though mine was about twice the price at $11.) I think mine holds 6 extra shells, and I always make the front two Brenneke slugs.
[This message has been edited by Long Path (edited August 18, 1999).]