I have a Time Capsule. It is... regrettable.
For one, swapping out/upgrading the hard drive is a PITA. (Have you ever torn a ligament? It sounds and feels about the same when you open up the TC. And oh yeah, bye bye warranty on that.) However, the mini won't help you in this regard.
Also, although I am a huge fan of the Airport Extreme, the concept of the TC - merging a router and a hard drive into one - really just doesn't have any advantages. The only one I can think of, which happens to be to only reason I'm keeping my TC at the moment, is that it allows you to have two separate network drives "connected" to a single Apple router. (I suppose if you are a very new newbie, then the idea of having it all-in-one poses as a good selling point.)
At least with the mini you are buying a machine that has practical uses. I'm not sure its best use is as a NAS, however. Other options: Just get an NAS enclosure. You can get one for a single-disk application or a big hefty expensive hot-swappable RAID box. (And don't waste your money on a Drobo.)