So far the only real advantages I've heard have been that its mesh network is more reliable than most typical home wifi-networks, and that it is cross platform.
There are many other positives mentioned throughout this thread, but some are not necessarily unique to Sonos vs. AirPlay. So here's a consolidated list, off the top of mind, just comparing those 2. I am purposely leaving out Bluetooth, Google Cast, Denon HEOS, Bose SoundTouch, and others like LG, Samsung, and Vizio that have at least dipped their toes into these waters as they are not the topic of most of the above discussion.
Sonos advantages:
1) SonosNet mesh network offers superior stability
2) Stupidly simple and effective -- perfect -- syncing among zones
3) Very strong customer support and return policy (if you've ever had AirPlay troubles, you know that you are very often on your own to troubleshoot as you as most likely using products from multiple-vendors, including possibly mixed brands of routers and speakers/receivers)
4) Cross-platform, with apps for iOS, macOS, Android, and Windows
5) Play series speakers (Apple does not have it's own line, 3rd party speakers addressed below as a positive for AirPlay)
6) App-based controller (not reliant on initiating device, system is controllable from any device on the network, consolidates media sources ranging from local library to Internet radio to free/paid streaming services)
7) Wireless 5.1 home theater system availability (acts as another zone for music as well)
8) Very simple to set up 2 Play speakers as a stereo pair for substantially better sound quality
9) Full company focus from Sonos, it is their 1 and only product, and they are by far the biggest player in the market. AirPlay, at least to me, is seemingly stagnant (I am surprised that Apple hasn't really leveraged AirPlay more for whole-home audio, it seems to be most promoted for use with AppleTV).
AirPlay advantages:
1) Built-in to many AV receivers and 3rd party speakers, so more "accessible"
2) Airport Extreme ($100) significantly less costly than Sonos Connect ($350) (most directly competitive Sonos component) and Airport Extreme is not just AirPlay receiver but also a router for extending wireless coverage
3) Tend to be more portable by design, so easier to take along to other locations
4) Ability to send all sounds from most apps (for those that don't like Sonos' single-app approach) from computer/tablet/smartphone (system sounds and YouTube being most notable, albeit often with some syncing delay)
Note that I will not simply dismiss Sonos as being "more expensive". That is not necessarily the case. Sonos might be more costly, and Sonos might be less costly, depending on the speaker/amp components needed. Here is a specific example I used a couple of days ago in this thread: As an example (and I am comparing list prices of relatively comparable setups): 2 Sonos Play:1s $400 (often on sale 2 for $350) vs. AudioEngine A2 speakers $250 + AirPort Express $100 (Side note: You would also have to have wiring between the 2 speakers as well as from the Airport Express to 1 of the speakers).
I think I've hit most of the main plusses for each. These differences probably won't get you running out to the store to replace a well-running AirPlay system. But if you are just building a new system, or ready to expand an existing wired system, Sonos is really tough to beat.
