At the risk of starting to feel more like a Sonos fanboy than the casually-satisfied customer that I am, I think it will be helpful to others considering a purchase to continue elucidating some of the differences between the systems.
While some are touting Sono's sound quality as better than the Airport Express, I am not sure because I haven't tested them against each other. However, the Airport Express has a digital out that is bit perfect with CD quality files (16/44.1). What this means is that it will send a bit perfect audio stream to your outboard DAC (receiver) for processing.
A DAC is a digital-to-analog converter. "Bit perfect" speaks
only to the digital side of that equation.
It means that the converter can and will utilize all of the data in the 16-bit/44.1khz stream, and that there's no data loss in the process of getting the digital data from a file on your hard drive to the entry point of the DAC.
This is all well and good, but it happens before the actual conversion to analog sound. Bit-perfect gets you cleanly to the entry "door" of the DAC, but what happens inside the DAC can result in different audio coming out the exit. Once you start talking about one DAC being better than another one, things get rather subjective, but it is certainly possible to demonstrate that a majority of people may prefer one to another. Ultimately, though, what matters is what you yourself hear and prefer.
I will note here that a truly bad DAC will be noticeable to anyone. But the gradation between good and great is much finer. Having listened extensively to both systems, I consider either one to have good quality for any non-obsessive casual audiophile. A/B test them if you're dead set on finding "the best", but either one will serve you with good sound quality (as far as the DAC is concerned).
Obviously, for video, Airplay kills the Sonos. The Sonos is only for music and Airplay works with audio and video.
For video it's a tie. In either system you'll need an AppleTV to Airplay video to your TV. In either system you can route the accompanying audio from the video stream.
The Airplay devices work with just about every audio app for the iPhone (I haven't run across an audio that doesn't work with Airplay in a very long time). The Sonos has built in apps, so it would not have the same library as iOS.
Sonos has built-in apps for a
large selection of streaming services, including Spotify, Rdio, Stitcher, Songza, Amazon Cloud, TuneIn, MOG, 8tracks, last.fm, etc.
Does iOS have more Airplay-enabled apps than Sonos has Sonos apps? Absolutely. I have some seriously niche-y audio apps that I am sure will never get a native Sonos app (check out Raudio). I do hope they will add Soundcloud and Bandcamp soon, though. But again, all you need is a $99 Apple TV to get those Airplay apps streaming into your system.
Also, there is an advantage to the built-in Sonos apps: they run from the Sonos devices, not from your iOS or OSX device. So, for example, if I am throwing a party and I wander out of range of the WiFi while I am outside checking on the grill, the music keeps going because it's running from the Sonos, not from my phone. This also saves battery life on your iOS devices, and enables nifty features like the Sonos alarm which reliably wakes me up to DKFM lite streaming radio every morning even if the battery is dead on every iOS device in the house.
Sonos won't integrate directly with your iOS music library, it is true. But it does integrate well with your iTunes library and playlists. The one caveat here is having to manually tell Sonos to sync the latest updates if you edit your playlists in iTunes. That is a little annoying, frankly.
Airplay is built into more products.
Absolutely true, and the fact that there is more competition in the Airplay-enabled speaker arena helps to keep their costs down. This one is a clear advantage.
Sonos is a closed system for a reason, though, which is the mesh networking explained above by notjustjay.
I can simply send the audio to the receiver and it will flip the receiver on and let me control the volume in the app. The Sonos, if you have one that can be hooked up to a receiver, will require you to turn the receiver on separately.
The Sonos system is designed to be receiver-less. And it's auto-on when audio is present, too. For example, if I Airplay something to the living room Apple TV, the speakers in that zone come on automatically.
Now, where one might run into issues is having existing equipment such as a receiver or pre-amp that doesn't support auto-on, which you want to integrate with the Sonos Connect products. But you'd have that same problem if you were doing an Airplay system as well. Neither system will magically make your beloved single-end-triode tube amp behave like a network-aware device.
The main upside to Sonos is that it creates its own network and that can be beneficial to people with old routers or other issues.
It can be beneficial to anyone with high network usage. Even if you're not getting audio drop-outs either way, there can be other benefits. The bandwidth not being used by the audio stream that is running locally on a Sonos speaker instead of streaming live can be used by Netflix to provide a higher streaming rate for an HD movie, for example.
Certainly, it is possible to build an Airplay system that practically never drops audio. But until Airplay implements a mesh-network-and-sync system like Sonos has, Sonos should always result in more free bandwith on your WiFi network for other tasks.
The other possible issue with Airport Express is the digital out may cause issues with some DACs. It doesn't send a signal between songs, so the DAC may have to reconnect and you could miss the first second of a song. I have several DACs and have never had the issue, but it does require research if you want to use it with a DAC.
Sonos has some similar edge cases, too. For example, you need to be very careful about your choice of TV if you want to build a home theater system around the Playbar. Not all TVs can decode a 5.1 stream from the Playbar's digital output. I did my research in advance and made sure my TV was supported, but it's something to be aware of. (Google around, there are threads where people post reports of TVs that are compatible for 5.1)
Also, the Apple TV2/3 are not bit perfect with 16/44.1. All audio is converted to 48khz. For audiophiles, this is a big deal. Most everyone else won't notice a difference.
I wouldn't even consider this a strike against the aTV. Upsampling (i.e. increasing the sample rate) is much less of a problem than downsampling, and as you say is practically inaudible for most people, just as expanding a 1024x768 photo to 1280x1024 doesn't result in any noticeable loss of fidelity.
What works so well with airport express is powered speaker systems you can pick and choose brand, size and sound you like. Also you may have some laying around you can use. I had a pair of JBL sound sticks with sub add airport express and that's my kitchen system. You can also pick up used systems on craigslist add airport express and save tons of money. You can also power much higher end speakers than Sonos.
Although the Sonos-designed speakers are the heart of their marketing for obvious reasons, you can use their Connect and Connect:Amp to connect to whatever super-high-end audiophile amps/receivers/speakers you may have (or whatever cheap stuff you can scavenge from Craigslist). The caveat here is that the Connect products are currently stereo-only, so you can use one for an audio-only zone, or to power the surround speakers of your choice in a home theater configuration. But you would still need the Sonos Playbar to do a home theater setup.
OK, finally let me tout one specific example where Airplay really shines: portable battery-powered speakers, which is a product category not currently served by Sonos. I have a Pioneer A4 Airplay/Bluetooth speaker for that.
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To sum up, let me say that I'd consider the entire Sonos system a non-starter if the AppleTV didn't exist. While Sonos has a decent variety of native audio apps, for us iOS and OSX users that is not enough. It's not a question of "Sonos or Airplay" but rather "Definitely Airplay! I want my video streams and all my iOS apps...but what system do I want to manage my actual listening equipment?"
The real question, for Apple users, is "Airplay alone, or Airplay and Sonos?" They work very well together, so for me the answer has been "Both" and I've been pleased with the results.
I hope this response has been helpful. I think now I should go help build a house for someone in need and let Sonos do their own marketing.
