Technical Rationale
Just to clarify, firewire enclosures use some sort of ATA (133/100/66) connection internally which is then briged over to the firewire bus (usually via an Oxford911 or 922 chip). For this reason a drive connected via firewire cannot be faster than the same drive over ATA/100. It might be the same speed, probably a bit slower due to latency added by the bridge chip. Plus, Firewire 400 tops out at a theoretical 400Mbits/sec which = 50MBytes/sec (actual bandwidth is a bit less), so the actual bandwidth IS less than ATA/100.
Of course all of this means relatively little since even the newest and fastest 7200RPM drives rarely achieve SUSTAINED transfers higher than 50MBytes/sec.
Therefore, in terms of drive performance, the differences are negligable. What it really comes down to is: do you want an external or internal drive?
-mcg