Kelly MC vs Marcinkiewicz 15
The Marcinkiewicz 15 has a wider cup diameter, the Marcinkiewicz 15 has a larger throat opening.
Cup Diameter
The Marcinkiewicz 15 has a cup diameter of 17.42 mm / 0.6858 in compared to 16.76 mm / 0.6598 in on the Kelly MC — a difference of 0.66 mm / 0.0260 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a very deep cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
The Marcinkiewicz 15 has a wider throat (4.57 mm / 0.1799 in vs 3.80 mm / 0.1496 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Material
The Kelly MC is described as Lexan polycarbonate, and the Marcinkiewicz 15 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.