Before buying my Airport Extreme I also considered to buy the Apple Time Capsule which is an Airport Extreme with one hard drive and one hard drive only. I ended up buying an Airport Extreme and a Synology DS211+ with two Western Digital 2 TB green instead. I am glad that I did not bought the Time Capsule because in my opinion it is only a router with a build in backup device.
The Time Capsule does not have the many features that an ordinary intelligent NAS has. You can not run services like FTP, HTTP (web server), use RAID and use surveillance.
I am a big fan of RAID and the lack of RAID configuration on the Time Capsule (There is only one disk) is extremely bad and this is intolerable for me. I want backup of my data with a RAID 1 configuration.
The Time Capsule can provide you with data back up but do not expect it to be the best solution in the wold. Go for a NAS with RAID.