Bach 18 vs Schilke 43A
The Bach 18 has a wider cup diameter, the Schilke 43A has a larger throat opening.
Cup Diameter
The Bach 18 has a cup diameter of 24.20 mm / 0.9528 in compared to 23.57 mm / 0.9280 in on the Schilke 43A — a difference of 0.63 mm / 0.0248 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 Schilke 43A has a wider throat (5.95 mm / 0.2343 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.