Like most aspects of the audio pursuit, personal preference varies. And of course what you are willing/able to spend. At one time I favored AirPlay over casting. But I have migrated back to the CCA and cast Tidal and Pandora Premium; I stopped my Apple Music subscription. Occasionally I listen to Amazon Unlimited. Who knows, my pendulum may swing back to the Apple camp some day! I still have the equipment and sometimes listen via AirPlay (not Amazon tho) as you can control the volume from the app.
For me, the biggest issue with Casting, besides the lack of services that it supports (which is my biggest issues with it), is the fact that it isn't built into any of my music playing components. I have three receivers and a Yamaha WXC-50 that have Airplay built right into them. What this means is that instead of needing a remote to turn on my receiver and switch to the correct input, I simply have the iPhone send audio to the receiver and it will turn it on and switch over to the Airplay input. It is just more convenient on a lot of levels. I could switch to CA when I am streaming from my NAS or use Google Music around the house (I am subscribed to Youtube Red), but it really just comes down to habit and with Airplay built into my daily processes, I never really think to use the CA or my other Casting devices. Also, with Google Play, I lose Siri (which only works with Apple Music) which is a feature I use nearly every time I listen to music from the iPhone.
However, I agree that it has a lot to do with the phone (or tablet) brand you use, how you listen, your wifi signal, and the services/apps you plan on using or plan on never using. There isn't just one good answer. It is smart to get all the information you can, though.
Personally, I am only using two Airport Express units nowadays since I have so many other things that support Airplay around the house. For some of my lesser setups, I have switched to Bluetooth (Amazon Echo Dot) since it has many of the same advantages of Airplay (works with any service I want) and connecting it to my phone is as easy as saying "Alexa, connect to phone". Just really convenient, not to mention that the Dot has Tune In, Pandora, and Amazon Music available without needing to connect it to my phone. Also, the Dot can be used to control Plex and J River in my other systems.
The Dot has also added multi-room audio, so it is getting better all the time. Its main downfall is the lack of a digital output and Bluetooth compression.