I have been wrestling with this over last couple of days since getting the WiiM Amp and pairing with some second-hand Q Acoustics 2050i'speakers...
I am a long time Spotify subscriber and have a family plan, so I have to keep it for the family whether I choose to use it or not... Yesterday I took out the one-month free trial for Amazon HD and was keen to test it to see the difference in streaming quality. Truth is, I struggled to hear any difference between the Spotify feed and the Amazon HD feed. In addition, I have a good look at both apps and concluded that the Spotify app on iOS, is far superior to the Amazon app and as a premium subscriber, I also get access to a loads of fee audio books on Spotify which I value...
So, my conclusion is that I really want one app for my music, podcasts and audiobooks and as Spotify ticks more boxes than other services, I'll probably stick with it and accept the downside of lower quality streaming through the WiiM Amp. However, i would love it if they introduced a higher quality service - which I'd pay more to subscribe to...