EE and CS are both going to be where the "rubber meets the road", or the application of these concepts, especially at the BS/MS level. Specifically in classes covering things like communication codecs, video/image processing, signal processing, and compression. If you're interested more in the foundations of these ideas, you really need to look more towards pure math. For instance, the beginning of every coding book I own starts with a review of abstract algebra, and lot of signal processing ideas are built on top of complex analysis.