I would say that the majority of jazz altoist do not use metal mouthpieces. There are always exceptions to the rule so don't take that comment as a judgement. I would suggest avoiding an extremely bright mouthpiece no matter what material it is made out of. The baffle (inside of the mouthpiece) is probably the most important area. This determines brightness, projection, and heavily influences the intonation. What inspired you to buy a metal mouthpiece? The few professionals that I know who use metal alto mouthpieces play more rock or fusion gigs than staight ahead jazz or big band.