Size of Drive
"However, the 1TB model seems seriously over-priced – just buy the 500GB model instead, along with a cheap USB hard disk if you need a bit of extra storage."
Note that Time Machine will not make use of an external disk, so if you want to keep a lot of data in Time Machine, get the 1TB model.
Also, the USB port is really useful for backing up your time capsule. This article makes no mention of that. Plug in an external 500/1TB drive and click, boom your time capsule is backed up to an external which you can take offsite should disaster strike at home.