I'm by no means an expert so someone who is by all means correct me, but I have asked very similar things in the past and based on those replies my understanding is that in terms of basic sound quality there ought not to be any difference. The BCC has a more modern bluetooth and I think Mozart is just their most modern streaming and web interface platform giving you more features and flexibility via the app (such as the ability to tailor the EQ). I'd be interested to know what, if anything else.

Both the BCC and the BSC are for streaming to. Neither can connect wirelessly to speakers on its own. In both cases for that you'd need the Transmitter 1. The T1 has a mode for 2.1 WISA speakers wherein it will manage the crossover between BL 18s and 19, as I understand it, leaving you only to set the sub level and distance/phase.  Whereas if you connect the BCC or the BSC via PowerLink you can daisy-chain B*C to one BL18 to the other BL18 to the BL19, but there is no crossover filtering meaning that the BL18s will output their full range in addition to the 19 outputting the bass. That overlap may make setting it up satisfactorily more difficult. Also it means that you lose one of the benefits of using a sub which is to alleviate your speakers of producing bass which should aid their performance and increase overall headroom before ABL or other protections kick in.

As to whether it sounds better via WISA or PL, I'll let someone else comment as I've not yet heard either.

I have my 18s running via optical.  I'd be interested in any experiences of WISA vs optical.

Perhaps someone could also confirm my understanding that you can't connect up 18s via optical and then run a power link off of one to a 19?