Regarding the availability of USB versus Firewire devices, they're really not in the same market since USB 2.0 is so damned slow.
USB is a misnomer anyway, it ought to be called USP. You can daisy chain firewire devices even if the devices have differing speeds provided the slower devices are at the end of the chain. You can't do the same with USB without a hub, unless you want the USB 2.0 devices even slower than before.
USB is more of a replacement for the parallel port than say SCSI.
Most high-end peripherals have a firewire version or are only available as firewire. Most drives have both ports.