The difference between these two sizes is the size of the mouthpiece rim, and cup depth. Both of these options we carry are small shank, which refers to the part of the mouthpiece that is inserted into the mouthpiece receiver. Small shank mouthpieces are the most common sold as this is what fits standard student lined instruments. Upgraded trombones and baritones may require a large shank.
 
6 1/2 AL is our most popular choice, and is a really great, medium sized mouthpiece that is quite versatile for all styles. This is also the choice of most of the band teachers we work with in Orange County. 
 
12C has a smaller rim and cup size. It has a slightly brighter sound, and may require a bit more effort to get a good sound working in the lower range.