The CM-311AE mic capsule is directly in front of the mouth with lips touching the foam windscreen. The mic has extremely high gain before feedback and isolation, so if feedback is a serious problem for you, the CM-311AE is a great choice. It's the mic used by Garth Brooks, Britney Spears, Janet Jackson, and many others.
The CM-312AE mic capsule is to the side of the mouth, and it has a hypercardioid polar pattern. It does not reduce feedback as well as the CM-311AE, but it works well for most performers. If you don't like having a mic right in front of your mouth, and feedback is not a serious problem, the CM-312AE would be the best choice. In other words, if you need your floor monitors to be very loud, choose the CM-311AE to reduce feedback. Otherwise, choose the CM-312AE. Both mics sound about the same.
You can order the models CM-311AESH or CM-312AESH, which have a connector already wired on that mates with Shure transmitters.