AirPort Express or Extreme?
It's an issue of what device you'll be using, how many of them will be connected simultaneously, if you need to stream music, etc. But based on what you said, here are a few suggestions:

In general, if you have a new Mac (mid-2013 or later, for most models), it probably takes advantage of 802.11ac, a spec which only the Extreme supports. However, if you want to stream music via AirPlay to speakers connected to your router (especially if your router is in a room you use a lot), and you're willing to give up some power, the Express might be a better choice. Another thing to note is that the Extreme allows for  both the connection of printers and external hard drives, should you wish to perform wireless file transfer/backup, while the Express only supports printers. If you plan on attaching Ethernet cables to your router, you may want the Extreme as it features four Gigabit Ethernet ports, whereas the Express has only two older-spec 10/100BASE-T ports. If you have a bigger house that needs better coverage, perhaps consider the Extreme.

For further reference, and a very useful comparison table, see Apple's "Compare WiFi Models" Page.