Bach 12E vs Bach Mega Tone Trombone 6½A
The Bach Mega Tone Trombone 6½A has a wider cup diameter, the Bach Mega Tone Trombone 6½A features a deeper cup.
Cup Diameter
The Bach Mega Tone Trombone 6½A has a cup diameter of 25.40 mm / 1.0000 in compared to 24.50 mm / 0.9646 in on the Bach 12E — a difference of 0.90 mm / 0.0354 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Bach Mega Tone Trombone 6½A has a deep 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.