The most noticeable differences are that the (C) model supports up to 300m transmission vs. 250m for the (B) model, the (C) model also supports 6KV surge protection and the (B) only supports 4KV, and the PoE budget of the (C) model is 115W which is more than the 100W of the (B) model.
There may be some other small differences that you can pick out from the datasheet but the above ones seem to be the biggest differences. (the physical design/layout of the ports has changed slightly as well)