Anyone know the differences?

The difference is in the layout.  The 178C has two live wells and the storage layout is different.   I thinks it's designed a little more with meat eaters in mind. (Crappie, Walleye,  etc)

obviously, one has a "c".

 

sorry, hahah..

On the 188 the 'C' changes the rod storage from center to port side.  On the 178 I think it's on the side for both models. The second live well takes space away from the gear storage.  Three across seating in front.  Great for crappie fishermen.  Not so perfect for bass, but I know a lot of guys that fish multi-species and they like the 178C and 188C.  

The C versions have slightly different layout, as already stated.

 

On the front deck there are two slots for chairs too, instead of just one on the standard models.