Bach 18 vs Kelly 18
The Kelly 18 has a wider cup diameter, the Kelly 18 features a deeper cup.
Cup Diameter
The Kelly 18 has a cup diameter of 32.77 mm / 1.2902 in compared to 32.10 mm / 1.2638 in on the Bach 18 — a difference of 0.67 mm / 0.0264 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 18 has a — cup while the Bach 18 has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Material
The Bach 18 is described as unspecified, and the Kelly 18 as Lexan polycarbonate. 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.