i have a wd120jb in an adaptec usb.firewire enclosure. i also have a ximeta netdisk 120 GB
firewire is fine, as is usb on both drives.
network connection on the ximeta is fast enough for 4x
depends on the chipset of the enoclosure and your internal usb as well. if you have multiple usb devices i would suggest firewire since usb bandwidth is based on the CPU and is shared among all usb devices which lowers each devices throughput. firewire is controller based and bandwidth is parallel so no sharing slowdown. firewire cards are very cheap