The answer is that it depends.
For having mocked the gods (don't ask) I was condemed, for a thankfully brief period of time, to make money editing wedding videos. (No, you don't want to know how bad that is. No, you REALLY don't want to know.) The only good thing to come out of it was I learned that USB and Firewire are not equal for video work: Firewire kicks its ass.
The problem is that when you're moving lots of data around, either pushing it out of your machine to do drive imaging, or sucking it in to do video playback or editing (MP3 playing is fairly low bandwidth), the
USB controllers, in my experience, saturate quickly. So all those fancy benchmarks saying the two are equivalent don't do sustained transfers. If you look at the comparisons, you'll find that Firewire consistently beats USB when you're doing moving lots of data. (Oh, and formatting was a lot faster with Firewire -- I tried it both ways -- which sort of surprised me.)
I sort of doubt that you'll have problems watching movies, because this is still fairly low bandwidth, all things considered. It's not like using a non-linear editing package (Premiere, Vegas, FCP) which can really stress out your disk bandwidth.
But, at the same time, you can get an external drive case which has both USB 2 and Firewire at a very small premium over the USB 2 only case, so there's no reason to not get Firewire. (SATA is comparable to Firewire, and the price of SATA drives hits parity with PATA all the time, at least when they're one sale or with a rebate...)
Buy one with both interfaces, try them out, and you'll know for the future.
