The Time Capsule is more robust for file sharing and time machine, especially for multiple drives. If you only have one drive, not so much of an issue, but it has only been recently Apple approved the newest APExtreme for time machine backups. The internal TC drive saves space. Wireless performance of the new generation seem to be about the same. The TC will get a bit warmer doing both TimeMach backups and wireless.
So the biggest advantage in a Time Capsule is that everything is in one box (and the HD cost doesn't seem as far out of line as previous versions).
There are many more advantages if you are buying previous versions.