Bach 8½A vs Kelly 5C
The Kelly 5C has a wider cup diameter, the Kelly 5C has a larger throat opening.
Cup Diameter
The Kelly 5C has a cup diameter of 16.76 mm / 0.6598 in compared to 16.10 mm / 0.6339 in on the Bach 8½A — a difference of 0.66 mm / 0.0260 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a very deep cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
The Kelly 5C has a wider throat (3.80 mm / 0.1496 in vs 3.66 mm / 0.1441 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Material
The Bach 8½A is described as unspecified, and the Kelly 5C 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.