I would recommend a Meyer 5M, but you may not like it because it has a medium tip opening. Since you're used to Guardalas you might want to try their alto mpcs. The most important thing is to try as many as you can, compare each, and make the best choice.
Well, I am making a trip to the Woodwind Brasswind retail store this weekend, that's the reason I'm asking about mouthpieces. It doesn't really matter what material, used to metal, but not opposed to hard rubber