Fun post! For me SL or BL, matters not. I have both and like them all the same. There is a sleekness and somewhat racer look to a SL or a BL with sideplates, especially with an English stock and splinter forend. BL's are just that, boxy looking, even with scalloped edges. Just can't beat a SL for looks!
As to which is better, who knows. A lot depends on the maker and the parts being used, proper hardening, polishing and fitting. I've seen some SL's that look wonderful on the outside, great engraving, the locks properly inletted and fitted, and on the inside rough as a cob. Makes you wonder how the gun could even fire! But, seen the same on BL's also.
Quality is the major factor IMO when dealing with either one. Still, a debate that will go one to the seconding coming (and beyond I bet).
Best!
Greg