The three-ring TSOB is the most secure system offered for the FA. A lot of folks, including FA, would recommend the three ring TSOB set up for the higher pressure guns like the 454.
If my plan was to shoot heavy loads most of the time I would go with the TSOB. If my plan was to shoot modest loads and shoot the heavy stuff every now and then, I would go with the Leupold set up and mount the scope forward till the front of the adjustment section of the scope met up with the front ring. Leupold says their scopes can be mounted in this fashion without damage. Many 454 FA owners use the Leupold set up in this way.
FA also offers a hip holster, scope exposed, for the two ring set up. If I were to carry a scoped handgun in a holster this is the style I would prefer.
Take Care,
Scott