Bach 1½GM vs Bach 1¼G
The Bach 1¼G has a wider cup diameter, the Bach 1½GM has a larger throat opening.
Cup Diameter
The Bach 1¼G has a cup diameter of 27.50 mm / 1.0827 in compared to 27.00 mm / 1.0630 in on the Bach 1½GM — a difference of 0.50 mm / 0.0197 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a very deep cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
The Bach 1½GM has a wider throat (8.10 mm / 0.3189 in vs 7.01 mm / 0.2760 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.