Marcinkiewicz 300 vs Marcinkiewicz 303
The Marcinkiewicz 300 has a wider cup diameter, the Marcinkiewicz 303 features a deeper cup.
Cup Diameter
The Marcinkiewicz 300 has a cup diameter of 17.58 mm / 0.6921 in compared to 17.20 mm / 0.6772 in on the Marcinkiewicz 303 — a difference of 0.38 mm / 0.0150 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Marcinkiewicz 303 has a shallow cup while the Marcinkiewicz 300 has a very 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 3.73 mm / 0.1469 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.