My advice is to stay clear of USB burners unless you are sure that you won't be using software that relies on the ASPI layer. ASPI for USB is often tied to the software provider (e.g. Goldenhawk for CDRWin) and if you use more than one piece of burning software that rewquires the ASPI layer, you could get into some compatibility difficulties. These difficulties generally don't arise with IDE or SCSI.

USB has other problems, such as shared IRQs or BIOS quirks in certain PCs and basically it's a can of worms if you want to anything more than copy music or backup PC data.