Bach 2½C vs Marcinkiewicz 1 / 5B
The Marcinkiewicz 1 / 5B has a wider cup diameter, the Marcinkiewicz 1 / 5B has a larger throat opening.
Cup Diameter
The Marcinkiewicz 1 / 5B has a cup diameter of 17.35 mm / 0.6831 in compared to 16.40 mm / 0.6457 in on the Bach 2½C — a difference of 0.95 mm / 0.0374 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a medium cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
The Marcinkiewicz 1 / 5B has a wider throat (3.73 mm / 0.1469 in vs 3.66 mm / 0.1441 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
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.