Bruno Tilz 305-4 vs Bruno Tilz 306-5 GS
The Bruno Tilz 306-5 GS has a wider cup diameter, the Bruno Tilz 306-5 GS has a larger throat opening.
Cup Diameter
The Bruno Tilz 306-5 GS has a cup diameter of 26.00 mm / 1.0236 in compared to 25.50 mm / 1.0039 in on the Bruno Tilz 305-4 — 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 medium cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
The Bruno Tilz 306-5 GS has a wider throat (6.80 mm / 0.2677 in vs 5.80 mm / 0.2283 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Which is better?
Both mouthpieces are categorized as "Catalog" — the physical dimensions above will determine which suits your embouchure and playing style better.