Chromecast is theoretically better than Airplay in that it sends the stream directly to the client, rather than using two wireless legs to stream to and from an iPhone, in addition to supporting up to 24/192.
In practice I’ve found it to be somewhat lacking in sound quality. Last time I tried it, I found the equivalent 16/44.1 stream to sound consistently better than a 24 bit stream of the same track via Chromecast. Still a good option for convenience in some situations, although using iOS, I tend to stick with Airplay.
This post by Steve Harris might explain a bit about how Chromecast compromises sound quality despite its hi-res ability: