Running the application benchmarks on external storage systems is certainly not a very pragmatic test, because very rarely would anyone install applications on an external drive, as they are not as good for this purpose as built-in internal drives.
However, WinBench 99 2.0 is a reliable way to measure the performance of a storage subsystem.
Conclusion: Excellent, Although Pricey
All of the components (USB, FireWire, and the hard drives we used) were highly developed products, so there were no problems to complain about during the test.
Installation is a standard process: attach the hard drive and insert the driver CD when Windows asks for it. It's not necessary to reboot under Windows 2000/XP, whereas Windows 98/ME requires this step. Finally, the new drive should be partitioned and formatted, so that it is indicated in Windows under its own drive letter.
Using each of these drives on a daily basis is about the same. If you need a problem-free backup mediam or a mobile storage medium, all three will suit your needs. However, it should be mentioned that, technically speaking, hard drives are not recommended as backup media, and that's why you should always back up your data on safe media, i.e., media that is not mechanically sensitive.
USB or FireWire? This question is not as easy to answer. Basically, both interfaces offer similar performance, but FireWire is not quite as versatile because it is not as widespread as USB. Here it should also be mentioned that the USB 2.0 standard is also yet to become fully mainstream. So when you decide on an interface, the application you plan to use it for should be the central factor: if real-time data transfer is important for you, then FireWire is the clear choice. Here, Western Digital made better performance scores than Maxtor.
Whatever you decide, you'll have to dig deep into your wallets: a high-quality external storage device is not to be had for under $300. For the FireWire drive from Western Digital, as well as for Maxtor's 160 GB monster, you'll have to shell out about $350. This is the price you pay for mobility, and even now, there's no getting around it.