Kelly MC vs Marcinkiewicz CH R 15S
The Marcinkiewicz CH R 15S has a wider cup diameter, the Marcinkiewicz CH R 15S has a larger throat opening.
Cup Diameter
The Marcinkiewicz CH R 15S 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 CH R 15S 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 CH R 15S 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.