Bach 10½C vs Marcinkiewicz E9M. Klein
The Marcinkiewicz E9M. Klein has a wider cup diameter, the Marcinkiewicz E9M. Klein features a deeper cup.
Cup Diameter
The Marcinkiewicz E9M. Klein has a cup diameter of 17.32 mm / 0.6819 in compared to 15.90 mm / 0.6260 in on the Bach 10½C — a difference of 1.42 mm / 0.0559 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Marcinkiewicz E9M. Klein has a deep cup while the Bach 10½C has a medium 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.