for me is the apple watch mostly. in second place the integration with orher apple devices. I know samsung and google have equivalents but none is as polished, reliable, seamless as apple. And in the smartwatch world, wear os is no match at all. When android will have a proper smartwatch and integration, i will happily switch to Android because i pretty much prefer it for my personal phone.
for me the biggest issue is that, if i stream a song or a video via airplay and want to look or hear something else on my phone, the stream stops and the new audio/video starts playing on the streaming device. Chromecast doesn’t have that problem, i can stream music from spotify to the home speakers and watch a youtube video on my phone, at the same time without interfering.