Bach 6½AL (large shank) vs Yamaha SL-48S
The Bach 6½AL (large shank) has a wider cup diameter, the Bach 6½AL (large shank) features a deeper cup.
Cup Diameter
The Bach 6½AL (large shank) has a cup diameter of 25.40 mm / 1.0000 in compared to 25.25 mm / 0.9941 in on the Yamaha SL-48S — a difference of 0.15 mm / 0.0059 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Bach 6½AL (large shank) has a deep cup while the Yamaha SL-48S has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
Throat diameters are essentially identical at 6.63 mm / 0.2610 in, so resistance feel will be comparable.
Which is better?
Neither mouthpiece clearly dominates the other — the right choice depends on your embouchure, playing style, and the sound you're after. Try both if possible.