I already have and am satisfied with Alexa, but HomePod interests me because of Apple's general stance towards privacy. Has there been extensive discussion on how Apple plans to differentiate Siri's data usage/sharing vs Google and Amazon?

But honestly, the main obstacle for me is Apple's lack of 3rd-party integration. For example, it appears that Siri can't be used to access Spotify. With Alexa, I can scope music requests with "Alexa, play some jazz from Spotify (as opposed to Amazon Music)". I can even configure Alexa to use Spotify by default -- e.g. prioritized over Amazon's own offerings.

I've a pretty large iTunes library from back in the day but I've completely stopped buying from iTunes (and haven't bothered to check out Music) because of lack of interoperability.