Bach 3C vs Schilke 15A4A
The Schilke 15A4A has a wider cup diameter, the Bach 3C features a deeper cup.
Cup Diameter
The Schilke 15A4A has a cup diameter of 17.14 mm / 0.6748 in compared to 16.30 mm / 0.6417 in on the Bach 3C — a difference of 0.84 mm / 0.0331 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Bach 3C has a medium cup while the Schilke 15A4A has a shallow 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 3.66 mm / 0.1441 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.