I've been playing both side by side for a few months and Castle Clash just surpasses CoC in nearly every way.

Updated graphics a couple patches ago are now both on par, but the number of activities you can do in CC are better, and the progress rate on CC is FAR better without being too much. You can also progress without spending real money much better in CC. Number of active people in CC is great too. The only point I'd give CoC is that it's more social, but gameplay definitely goes to CC