Bach 12E vs Schilke 51C4
The Schilke 51C4 has a wider cup diameter, the Schilke 51C4 has a larger throat opening.
Cup Diameter
The Schilke 51C4 has a cup diameter of 25.63 mm / 1.0091 in compared to 24.50 mm / 0.9646 in on the Bach 12E — a difference of 1.13 mm / 0.0445 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a shallow cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
The Schilke 51C4 has a wider throat (7.04 mm / 0.2772 in vs 5.85 mm / 0.2303 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.