Equalizer,
Not sure if I am interpreting your question differently from the other responses above, but I guess you are asking about USB vs. FireWire audio cards.
You can get a USB card for a lot less than FireWire, for sure. The thing to keep in mind is whether you need a single output or multiple ones. All of the FW cards on the market have multiple inputs and outputs, all sent as individual digital streams between the card and the computer.
The USB cards are all based on USB 1.1 standard (with one exception of Edirol UA1000(?) ). The USB 1.1. is about 35 times slower than FireWire, and it only allows for carrying one pair of stereo inputs and one pair of stereo outputs. Whenever you see a multi-input USB 1.1 based audio card, the inputs are all mixed down into a single pair in the card/breakout box, and the USB wire carries only a single stereo signal in (and a single stereo signal out). This standard is quite dated (at least 7 years old).
There is a new standard of USB 2.0, which has a bandwidth (speed) comparable to that of FireWire, and actually, a bit higher. It can support multi-channel audio I/Fs, where each stream is digitized and carried individually to the PC (or from the PC to the card). Most if not all new computers have USB 2.0 ports on them, and if they don't you can buy a cheap adapter (<$50). However, the USB consortium had trouble agreeing on some audio capabilities for a long time. As a result, AFAIK only Edirol is currently making USB 2.0-enabled sound card, which is also expensive - quite comparable with the FireWire ones.
It is true that unless you are heavy into recording and don't need individual outs or ins (beyond one pair of ins and one pair of outs), the USB 1.1-based card will work fine (and USB 1.1 based devices do work when connected into USB 2.0 ports on your computer). Most good cards will produce uite little latency, making it suitable for live playing.
Regards,
Alex
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Regards,
Alex