Not sure that Time Capsule transfer speeds would be as fast as FW800. But the biggest disadvantage, IMHO, is that it cannot be used as a boot volume. You need the Airport parts up and running before you can access the drive and that doesn't happen until quite late in the boot up routines, apparently. The price is not too far off for a 1TB external drive and it does have the Apple type plug 'n play, use setup but external drives are easily handled/accessed by the OS (even Tiger), anyway. My setup was destroyed near the end of March so my current backup is only about 45 days. It is 72.6+ GB. But I don't do video or music work as you do so you'd likely have a somewhat larger back up in the same time frame.
The biggest advantage seems to be the extra ports and the Airport router that could be used to connect other Macs in the house. Of course, most external housing can be bought with extra FW and USB ports, also.