Bach 1¼G vs Bach 6½A (large shank)
The Bach 1¼G has a wider cup diameter, the Bach 1¼G features a deeper cup.
Cup Diameter
The Bach 1¼G has a cup diameter of 27.50 mm / 1.0827 in compared to 25.40 mm / 1.0000 in on the Bach 6½A (large shank) — a difference of 2.10 mm / 0.0827 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Bach 1¼G has a very deep cup while the Bach 6½A (large shank) has a deep 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 7.01 mm / 0.2760 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.