Wookieepedia and the canon source books have not helped me in this matter. The TC-series and C=series protocol droid seem to be the same height, and look the same. Is there a difference between them that I'm missing?
In Legends, TC-series droids were a protocol droid line produced by Cybot Galactica. They closely resembled the 3PO-series* protocol droids, and were sometimes referred to as TC-3PO protocol droids. The main difference was that TC-series droids were mostly created to serve on space stations, whereas 3PO-series droid were more environmentally eclectic. (It should be noted that due to his rebuild, C-3PO more closely resembled a TC-series droid than a 3PO-series droid)
We have not seen an appearance of these droids in Canon since the decanonization of the EU, and the above difference was never explicitly mentioned on screen (The Phantom Menace and Star War: The Clone Wars, both of which are of course is still Canon). So currently there is no practical difference and they can be assumed to be different versions of the same basic model.
*The droids you call "C-series" are actually 3PO-series droids. I don't know why.