Kelly 6-1/2AL-LS-SS vs Yamaha SL-48DL
The Yamaha SL-48DL has a wider cup diameter, the Kelly 6-1/2AL-LS-SS features a deeper cup.
Cup Diameter
The Yamaha SL-48DL has a cup diameter of 25.25 mm / 0.9941 in compared to 25.15 mm / 0.9902 in on the Kelly 6-1/2AL-LS-SS — a difference of 0.10 mm / 0.0039 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 6-1/2AL-LS-SS has a very deep cup while the Yamaha SL-48DL has a deep cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Material
The Kelly 6-1/2AL-LS-SS is described as 316 Surgical Stainless Steel, and the Yamaha SL-48DL 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.