Curry 3 BBC. vs Kelly 3C
The Kelly 3C has a wider cup diameter.
Cup Diameter
The Kelly 3C has a cup diameter of 17.02 mm / 0.6701 in compared to 16.89 mm / 0.6650 in on the Curry 3 BBC. — a difference of 0.13 mm / 0.0051 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
Throat diameters are essentially identical at 4.06 mm / 0.1600 in, so resistance feel will be comparable.
Material
The Curry 3 BBC. is described as unspecified, and the Kelly 3C 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.