I actually use Airport Extreme instead of Airport Time Capsule. The real difference is ATC has a hard drive built in while AE does not (you connect an external HD). I prefer this because of modularity, if either of the components in the ATC breaks (wifi or HD) then you're sort of out of luck. It made a little more sense for me to get AE because I already have a ton of externals I can use. You can swap out externals and choose the size that works for you.
Either way, they're terrific little units. Wifi is better at my house and I sync my time capsule with it and it does everything behind the scenes. Like most things Apple, it was a breeze to setup.