ATA100 max is 100 MB/s and probably a little lower. Firewire theoretically maxes at 50, but the fastest real life speeds are <35 MB/s. And that's with the Oxford 911 chipset - others are much slower. No drives hit 100 MB/s, but many go over 35 MB/s.
As for internal Firewire, I have one internal Firewire port, but it doesn't make a lot of sense to use it because the drive is IDE anyway, and you have to use a US$60 Firewire-IDE bridge board. OTOH, if you've already filled up the IDE controller and have an extra internal Firewire port, Firewire might make sense.
Firewire is way better than both SCSI and USB though for external drives.