Speed has nothing to do with it. FireWire 400 and USB 2.0 have more than enough bandwidth for any feasible number of channels. The problem with USB is that the bus controller is exceptionally dumb, leaving it up to the CPU to do all the work of copying data around. By contrast, FireWire has support for DCL programs that run on the FireWire chipset itself, so (assuming the driver is written correctly) the CPU doesn't get involved until much later in the process.
This is a significant win since the most critical part of the handling of audio data is copying the data from the interface to an in-memory ring buffer with sufficiently low latency to avoid pops and crackles. As a result, for high-end audio, there is no substitute for FireWire. USB 3.0 is no better in this regard.
The only question is, "Is USB good enough?" And the answer to that depends in large part upon how fast your CPU is, whether you are doing projects with large numbers of tracks, whether you are using internal or external hard drives (in particular, USB for audio drives is a non-starter because USB doesn't have true DMA support), and how many tracks you intend to record at the same time. A good rule of thumb is to not go past about two channels with USB 1.1 interfaces, and not go past about eight channels with USB 2.0 interfaces.
Also, be aware that not all USB interfaces are created equal. There are a number of broken USB 2.0 interfaces that use the USB 1.1 audio spec over a USB 2.0 pipe. If you can avoid those, you should do so. Look for a class-compliant USB 2.0 interface. In the long run, you'll have fewer problems if your interface is USB 2.0 audio-class-compliant.
BTW, I don't know why folks seem to think FireWire is "fading fast". In the consumer space, it is declining, but that's just because the average consumer doesn't care about performance; the average consumer only cares about initial purchase cost. That's not the case in pro audio or pro video, and FireWire is still quite strong (and growing) in those areas last I checked, though USB is making a few inroads (with varying degrees of success). Of course, for disks, eSATA has an edge in both price and performance (and always will) over FireWire, and a HUGE edge over USB (even 3.0), so if you're thinking about adding external disks, go eSATA if you can.
Im looking for an interface within about $300. I dont really know what firewire is or if my laptop has a port for it. I read USB had more bandwith, but it shares it with whatever else is on the USB card. I wont have anything else plugged in to USB, but i dont know if other stuff is connected internally to the card that would make it slower. Any help or recommendations would be great! Thank you
