BruceRThe 215s and 315s are both entry-level Shure IEMs. They are roughly in the same bracket. The 215s use a single dynamic driver, while the 315s use a single balanced armature driver. Neither one is "better" than the other. The 215s are going to have more bass (due to the nature of dynamic drivers), while the 315s are going to have better treble and probably a bit better detail retrieval (the 215s have more bass, but severely lack in the mids and treble).
If you really want a single BA driver, you can't go wrong with Sony's XBA1, which can be had for $25. It doesn't have removable cables, and it's not going to isolate nearly as much as the Shure, but it also costs like 1/5th the price.
BruceRI just bought some 215s based off this thread, I have to say they are very fun as stated. They are putting my SE530s to shame with the low end, even though it's not quite as tight.