I’ve got a Bb baritone that takes large shank mouthpieces. Should I get a 3g or 4.5al?
3G is a big mouthpiece. Between the two, I'd go for the 4.5AL. But there are plenty of other choices, and the expensive 'name' brands don't really contain any special magic! What are your fellow baritone players using? What else have you tried?
When I was a student (and, apparently still today) the fashion was to play the biggest possible instrument, with the largest mouthpiece, in a quest for a 'big' heavy sound. This is not always the best idea! When EVERY brass instrument is trying to sound like a tuba, what's the point? :-)