As mentioned above, USB will probably last you a lot longer considering Apple is dropping FireWire 400 support from its machines. You can only get 800 these days on the more "professional" models. FireWire 800 is very fast for moving gigabytes of files for the most part. However, since I have more USB drives around, I keep all of my video on those. USB is quite tolerable when moving a couple of hi-res videos, even when going through a hub. I've had no problem copying files out of iTunes, Final Cut Express, or iMovie.
If you plan on moving iMovie files around, be careful with those project files. iMovie '08 is very specific when storing external video files, but naming a new folder called "iMovie Events" should solve that, according to the program's built-in help section. I haven't upgraded to '09, so I can't say the same for the latest version should you have it.
Bottom line: if you have the extra money and a capable Mac, FireWire is a good way to go, but know that FireWire support may be dropped in the not too distant future. Apple's trying to go with what's favored by the masses, and with the purported speeds of the new USB 3.0 specs, can't blame them too much. USB 3.0 will have up to (theoretically) 4.8Gbps. That's pretty fast. Not much has been said about FireWire 3200, on the other hand. In the long-term, USB will be worth it in terms of compatibility.