Marcinkiewicz 6 / 10.5C vs Schilke 13A4a
The Marcinkiewicz 6 / 10.5C has a wider cup diameter, the Marcinkiewicz 6 / 10.5C features a deeper cup.
Cup Diameter
The Marcinkiewicz 6 / 10.5C has a cup diameter of 16.84 mm / 0.6630 in compared to 16.76 mm / 0.6598 in on the Schilke 13A4a — a difference of 0.08 mm / 0.0031 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Marcinkiewicz 6 / 10.5C has a medium cup while the Schilke 13A4a has a shallow cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
Throat diameters are essentially identical at 3.66 mm / 0.1441 in, so resistance feel will be comparable.
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.