Bach 12E vs Kelly 6-1/2AL
The Kelly 6-1/2AL has a wider cup diameter, the Kelly 6-1/2AL features a deeper cup.
Cup Diameter
The Kelly 6-1/2AL has a cup diameter of 25.15 mm / 0.9902 in compared to 24.50 mm / 0.9646 in on the Bach 12E — a difference of 0.65 mm / 0.0256 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 6-1/2AL has a very deep cup while the Bach 12E has a shallow cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Material
The Bach 12E is described as unspecified, and the Kelly 6-1/2AL 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.