AFAIK, USB2 speed is 480Mb/s and FW is 400 or 800 Mb/S.
Protocol-strictly speaking, FW800, if supported by your mac and your external hd, is a better choice in terms of efficency.
USB2 is widely implemented on PCs, therefore it could be interesting to be able to be connected to such computers if needed.
If it exists a usb2/FW external hd gives you the most compatible solution. It depends on how you expect to use your hd.But I'm not sure of the performance of dual interface hds (the more you do, the badder it is)🙄
As far as I'm concerned, i've bought a mini fw hd (40 GB Archos). My ibook supports only FW400, but my aim is to store video projects (imovie & idvd), which represent a average of 6 to 10 GB per project. However, it's efficiency is quite bad : it scores 22 on Xbench, although my internal hd scores 53. But it's ok for my needs.
Hope I've given you a few useful elements.