It is also a matter of what is comfortable for the student. Some students have thinner lips or thicker lips and some students use more air or less air which makes a difference in which mouthpiece to use. In general, thicker lips should have a larger rim size like the 51 / thinner lips should have a smaller rim size like the 6.5al. If they use a lot of air, the cup size should be bigger like the 51D, less air would work better with the 6.5al.
Both mouthpieces can sound good in the hands of different players. Also, the horn they are using with respond better to each of those pieces. Larger horns like bigger mouthpieces (Bach 42B) whereas smaller horns like smaller pieces (Conn 88H).
I know this doesn't really help too much but hopefully you can look at your students and help them find what fits them.