a) Siri. With the existing Music app I can tell Siri to play a particular artist or playlist. I would expect far deeper integration.
b) Watch. The Music app is native, has a glance.
c) Desktop. Spotify client on the desktop is slow and clunky. iTunes on the Mac is anything but.
On another note, Apple have a pretty good story with Airplay - does Spotify on iOS support that fully? On Android they don't support Chromecast and I've always thought that was an advantage of Play Music over Spotify - maybe there's an analogous situation on iOS?
On the other hand, Spotify has its own streaming protocol which is supported by a few player/speaker brands (e.g. Gramofon).
a) and c) are only relevant to Apple users. Spotify pretty much runs everywhere.
b) Watch for many is just a gimmick. This is really weak.