Kelly 24AW-SS vs Kelly 25
The Kelly 24AW-SS has a wider cup diameter, the Kelly 24AW-SS features a deeper cup.
Cup Diameter
The Kelly 24AW-SS has a cup diameter of 31.24 mm / 1.2299 in compared to 30.99 mm / 1.2201 in on the Kelly 25 — a difference of 0.25 mm / 0.0098 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 24AW-SS has a very deep cup while the Kelly 25 has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Material
The Kelly 24AW-SS is described as 316 Surgical Stainless Steel, and the Kelly 25 as Lexan polycarbonate. Material affects mass, warmth, and how quickly the piece responds.
Which is better?
Both mouthpieces are categorized as "General" — the physical dimensions above will determine which suits your embouchure and playing style better.