I think the FR and the SB have some great things in common, and have some good differences. The things in common are the Balvenie house style. Both share a rich honey and vanilla taste. I think the wood is there, but it does not overwhelm the whisky. They both are well balanced (99% of the time with the SB). Some of the differences: 47.8% ish ABV for the SB, 43% for the FR. The FR has a consistent profile, where as the SB usually only has small variances but it is different cask to cask. Lastly the FR, to me, has a little hint of smoke on the taste. The whisky finnishes with a lingering taste of smoke. I haven't really seen that much in the SB's.
I love them both, so IMHO you cant go wrong.