TOP 10 DIFFERENCES
The newer Airport Express typically sells for $90 or less (the older version is uglier but can typically be found for around $40). It supports digital and analog via a mini port as well. However, it can also expand your wireless network, has an ethernet input and output, and a USB port for connecting a printer. It supports Wireless N.
The newest Airport Express is a small square (with rounded corners) white box that takes up more space. than the CA. However, in my opinion, it looks good enough to have on display.
The photo below is the back of the newer Airport Express:
The Airport Express is limited to 16/44.1. However, it is bit perfect rather than resampling to 48khz, which is what happens right now with any audio I send to the CA.
The Airport Express uses Airplay and has little support on Android, but is supported by virtually every app on iOS. Since Airplay is built into the iOS operating system, it can be accessed even when an app doesn't have direct support.
Airplay sends audio to the mobile device and then the mobile device sends audio to the Airport Express. This means that you will lose the audio stream when you take a phone call. It also means that if you have an outdated router, you could be more susceptible to drop outs.
Airplay supports multi-zone audio now, but it has some limitations (see my article on this subject).
With an app called Airfoil, you can stream any audio signal from your computer to any or all Airplay devices. Also, iTunes has multi-zone Airplay built into its normal interface.
With the Airport Express you can exclusively be on ethernet. You simply send the audio signal from the service to your connected computer and “Airplay" over a wired connection to an Airport Express that is also wired.
With Airplay, you can split the signal and watch the video on the iPhone and hear the audio on the Airport Express.
The Airport Express cannot be powered by USB. Also, the audio tone that lets you know the CA is connected is unusually annoying. If you are just hooking up the CA, make sure you turn that off in the settings.