I have both devices but in this case it would be Chromecast Audio. I currently get by with casting my screen but native Chromecast support in Bandcamp would be much better - music would go directly from bandcamp.com to Chromecast Audio as opposed to bandcamp.com to my phone to Chromecast. Would save a lot of phone battery and the extra decode + reencode on phone that happens in casting the screen.
This is something I'd really like too, I have the speakers in my apartment set up with a pair of Chromecast audios, and would use the bandcamp app a lot more if it could work with them. At the moment I just try to find the bands on Google music instead, because that does work.