Acans are more 'puffy' and I see them as separate entities, where favia is sort of a colony. I've often wondered if they food eaten by one 'head' of favia doesn't in fact feed the whole colony, where acans most likely just feed themselves.
The great thing about lps is that you can sort of control growth WAY better than softies. Softies are sort of weeds, favia is a colourful encruster, it can make your rock look magical, and slower growing and less aggressive than say a montipora. If you want it to slow its growth, stop feeding it