|First of all, USB-2 is faster than ANY IDE OR SCSI CABLE!!! The only limitation is the speed of the drive itself, the interface (ie cable/chipset) is superior.
Actually, USB-2 is much, much slower than faster SCSI, and the fundamental technology behind FireWire is superior (in fact, the best SCSI is still far superior to even the current FireWire spec in terms of pure data transfer rates). Even good IDE connections are faster than the current implementation of USB-2.
USB and USB-2, like IDE drives but even more so, are highly dependent on your computer's processor. Doing anything else at the same time can significantly slow down the transfer rate. Since SCSI and FireWire have their own independent controllers built in, their data transfer rates are higher, even at the same spec (i.e. 400Mb/s vs. 400Mb/s). Another bonus for FireWire is that it is device independent -- you don't need a host computer to connect. You can actually directly hook up two video cameras. FireWire also has the ability to broadcast, which USB-2 doesn't.
USB-2 also has other drawbacks. The bandwidth of a USB-2 connection is split into fixed proportions -- if you have two devices attached, each can only get the maximum of 1/2 the total bandwidth, even if the other is idle. So if you have a mouse and a hard drive on a USB-2 bus, they each get half the bandwidth, even though the mouse obviously doesn't need more than a fraction of it. Some analysts have forecasted that "real-world" use of USB-2 bandwidth could be around 58Mb/s. Also, if you introduce a USB 1.x device into a USB-2 chain, the whole chain can slow to USB 1, depending on the device and controller. Finally the "cheaper" cost of USB-2 (as compared to FireWire) isn't as cheap as you might think, since in order to be backwards-compatible with USB, USB-2 devices need to incorporate multiple chipsets -- USB-2 itself is not backwards-compatible.
The other hurdle that USB-2 will have to overcome as a high-speed data transfer connection is the fact that it also has to allow slower devices to co-exist. USB-2 is Intel's way to replace *both* USB and FireWire, so it can be used with both low-speed and high-speed devices. Under USB 1.1, 90% of bandwidth is guaranteed for isochronous transfers (data from sources such as video and audio controllers), while only 10% of the bandwidth is guaranteed for bulk transfers. Of course bulk transfers can *use* much more, but in order to ensure that "more important" data (such as mouse movements and video) has priority, the 10/90 guaranteed bandwidth rule was devised. If this holds true under USB-2, you may never see the promised speeds of 400Mb/s.
What it comes down to is that USB was designed to be a convenient replacement for serial, PS/2, and, to some extent, parallel ports -- low-speed connections. FireWire was designed from the start to be a convenient, high-speed connection for large, fast transfers of bulk data. USB-2 is an attempt to make FireWire out of USB, but unfortunately it's not working. It will probably end up being widespread, but that's more because it's an Intel technology that they can throw on motherboards for cheap than because it's actually a good technology.
As for right now, FireWire is much more widespread. Even Microsoft considered not supporting USB-2 natively. FireWire did start on Macs, but it is quickly becoming the standard for A/V data transfers, with every modern digital video camera and every Sony computer sporting FireWire ports. Many other PC manufacturers offer it as an inexpensive option.
Oh, and FireWire will be at 1600Mb/s pretty soon