Bach 12E vs Yamaha SL-45C2S
The Bach 12E has a wider cup diameter, the Yamaha SL-45C2S features a deeper cup.
Cup Diameter
The Bach 12E has a cup diameter of 24.50 mm / 0.9646 in compared to 24.26 mm / 0.9551 in on the Yamaha SL-45C2S — a difference of 0.24 mm / 0.0094 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Yamaha SL-45C2S has a medium cup while the Bach 12E has a 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 5.85 mm / 0.2303 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.