I have always used a parallel port for my printers. My first one didn't have USB, the onbe I have now does, but I use the parallel port since:
1. Is it mainly for a printer, I have nothing else that would/could us it.
2. Why tie up a USB port when the other decicated port is there and I have the cable?

BUT, is there:
1. Less processor usage using a USB port?
2. Faster data transfer (USB 1.1 that is)?

I notice when I send a larger 'job' to the printer it really 'chokes' the processor, maxes it out until the job is done. I don't know if it is a port issue or something else.

Specs:
2k, w/sp4 14 processes running at startup
AMD 2100b Throughbred
nForce2 MB
512 MB PC2700
512 MB PC2700

You use more CPU when using an USB port.
USB port is faster than parallel but most printers are usually slower than a parallel port in EPP mode.
If you have a PostScript or PCL printer then you may benefit from USB transfer rates if you are printing vector graphics.

Make sure you have EPP enabled in BIOS for the parallel port and enable bidirectional etc. support in printer settings also.
Also, you can set the spooling priority in printer settings. Lowering that may make things smoother.