Bach 2½C vs Kelly 7C-UK
The Kelly 7C-UK has a wider cup diameter.
Cup Diameter
The Kelly 7C-UK has a cup diameter of 16.51 mm / 0.6500 in compared to 16.40 mm / 0.6457 in on the Bach 2½C — a difference of 0.11 mm / 0.0043 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 2½C is described as unspecified, and the Kelly 7C-UK 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.