I have used various combinations of hardware (Apple TV, Amazon Fire TV, Sonos, Naim SuperUniti, Argon Stream 2, PC lap Top, Chord Hugo TT) to get the best SQ and useability from streaming Spotify to a HiFi whilst using an iPhone to control and so hopefully my findings below will be helpful.
To get the best SQ from Spotify you need to use a device with Spotify Connect functionaltiy and send the digital siginal from that piece of hardware into a high quality DAC (if it doesn't already have one).
I have previously used Apple TV as my streaming device using Airplay and the digital out to an external DAC. This is the most pratcial fix for you if you want to stream both Spotify and Apple Itunes from your iPhone. The SQ for Spotify via Airplay is OK but using a device with Spotify Connect does make a noticeable difference in detail and clarity.
I part exchanged a SuperUnti (which also has Spotify Connect) to buy a Chord Hugo TT and bought an Amazon Fire TV (which has Spotify Connect) to use as the streamer into the Hugo. It was by far the best SQ from Spotify I have heard however I found that you really needed the AFTV to be permanently hooked up to a TV/monitor to be able to get it to talk to your iPhone (it would disconnect periodically). I don't have aTV near my Hi-Fi so this was an issue. Also, AFTV does not allow you to remotely control the volume of Spotify Connect from an iPhone when using the digital toslink out, so you would need to have a remote control for the volume on your amp. This issue combined with the TV element meant i sent the AFTV back and instead bought an Argon Stream 2 which is a fairly cheap (£120) streaming device with Spotify Connect and DLNA. So far it has worked well with Spotify Connect and allows volume control from the iPhone. The Hugo TT is a fabulous DAC and, even when combined with the cheap Argon Stream 2, provides SQ which is streets ahead of the Naim SuperUniti/Spotify connect option.
The Argon deos produce a slight "tick" noise when selecting different tracks from the iPhone which may be down to the fact that it came from Germany with a two pin European plug and i have added a converter. It's not a deal breaker for me at this price.
When it comes to discovering new music, no other service comes close to Spotify and so I would say that a Spotify Connect enabled sreaming device is the best way to go.