Bach 12C vs Kelly JAZZ-LS-SS
The Kelly JAZZ-LS-SS has a wider cup diameter, the Kelly JAZZ-LS-SS features a deeper cup.
Cup Diameter
The Kelly JAZZ-LS-SS has a cup diameter of 25.65 mm / 1.0098 in compared to 24.50 mm / 0.9646 in on the Bach 12C — a difference of 1.15 mm / 0.0453 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly JAZZ-LS-SS has a — cup while the Bach 12C has a shallow cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Material
The Bach 12C is described as unspecified, and the Kelly JAZZ-LS-SS as 316 Surgical Stainless Steel. Material affects mass, warmth, and how quickly the piece responds.
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.