Kelly 3C vs Kelly 7C
The Kelly 3C has a wider cup diameter, the Kelly 3C features a deeper cup, the Kelly 3C has a larger throat opening.
Cup Diameter
The Kelly 3C has a cup diameter of 17.02 mm / 0.6701 in compared to 16.51 mm / 0.6500 in on the Kelly 7C — a difference of 0.51 mm / 0.0201 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 3C has a very deep cup while the Kelly 7C has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
The Kelly 3C has a wider throat (4.06 mm / 0.1598 in vs 3.66 mm / 0.1441 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Material
Both mouthpieces share the same listed material (Lexan polycarbonate).
Which is better?
Both mouthpieces are categorized as "General" — the physical dimensions above will determine which suits your embouchure and playing style better.