SMS and MMS are different types of messaging mechanisms. SMS (Short Message Service) is delivered through the cell radio bearer channel and is just a text string while MMS (Multimedia Messaging Service) is data and can be text, images and sound. It needs APN settings and internet access.
MMS has the greatest potential for exploitation which can be mitigated by not having your Messaging App to auto-download messages and I think this is a good solution for those unexpected MMS messages like spam.