Which Apple router

with the extreme you can easily use it for time machine backups just by adding a standard external USB hard drive. you can add a USB printer too. (adding more than one device will require a USB hub)

the express USB port can only be used for printers. not disk drives.
the express adds the ability to plug speakers into the express, and send audio from your comptuer or iOS device to those speakers.

 

Get a extreme on ebay or amazon. You can get this same $100 model for 50-60 bucks

 

Only the current model Extreme is supported for Time Machine backups. (Restoring from an older-model Extreme is iffy. Not recommended.)

And that's marvelous, as it separates the storage from the router. I love my Time Capsule but dread the inevitable hard disk failure that will necessitate a whole new router. The new Extremes don't have that issue.

Take an Extreme, connect a USB hub and a couple of good-quality USB drives, and you can do alternating backups automatically. Superb.

(A powered hub will be needed if the drives are bus-powered.)

http://tidbits.com/article/14347?rss provides a great discussion of this new capability. It deserves to be major news.

 

For basic wireless router duties, get either and you're likely to be very happy with it.

Get the Airport Express: to use AirPlay to play music from any Mac, iPod, or iPhone on your network to a stereo system or powered speakers.

Get the Airport Express: to same some money.

Get the Airport Extreme: to plug in a USB hard drive for shared network storage. (Apple doesn't, and I don't, recommend you use this for Time Machine backups though--you should have the latest "tall" version of the Extreme or any version of Time Capsule for that.)

Get the Airport Extreme: to have a "Bonjour Sleep Proxy". If a Mac on the network is providing services such as Remote Screen Sharing, Remote Login, iTunes sharing, etc., this allows the Mac to go to sleep. When another Mac, iPod, or iPhone wants to use the services, the Airport Extreme will wake the serving Mac to provide them, after which it can go to sleep again. This is really nifty but probably not something you'll need for a very basic home setup. (See http://support.apple.com/kb/HT3774?viewlocale=en_US&locale=en_US

They are both easy to configure. They both provide solid, reliable, WiFi networks with simultaneous dual-band.

 

I think the ML requirement comes from setting up the network, not operating it. A Wi-Fi is a Wi-Fi. Standardized.
You need ML to run AirPort Utility version 6.x to set the thing up.
AP Utility 5.x (compatible with earlier OS-s), won't find the new basestation.