Kelly 7C vs Monette Cornet AP2.5 S3
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 16.40 mm / 0.6457 in on the Monette Cornet AP2.5 S3 — 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 Kelly 7C is described as Lexan polycarbonate, and the Monette Cornet AP2.5 S3 as unspecified. 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.