Kelly 6-1/2AL-LS-SS vs Marcinkiewicz 11
The Marcinkiewicz 11 has a wider cup diameter, the Kelly 6-1/2AL-LS-SS features a deeper cup.
Cup Diameter
The Marcinkiewicz 11 has a cup diameter of 25.40 mm / 1.0000 in compared to 25.15 mm / 0.9902 in on the Kelly 6-1/2AL-LS-SS — 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 6-1/2AL-LS-SS has a very deep cup while the Marcinkiewicz 11 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 Marcinkiewicz 11 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.