Bach 2C vs Kelly 7C
The Kelly 7C has a wider cup diameter.
Cup Diameter
The Kelly 7C has a cup diameter of 16.51 mm / 0.6500 in compared to 16.50 mm / 0.6496 in on the Bach 2C — a difference of 0.01 mm / 0.0004 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a medium cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
Throat diameters are essentially identical at 3.66 mm / 0.1441 in, so resistance feel will be comparable.
Material
The Bach 2C is described as unspecified, and the Kelly 7C 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.