USB will send all the data to the printer immediately and the printer starts to print. Parallel is slower so it sends page 1 and the printer starts to print, and while page 1 prints it sends page 2, then page 2 prints and it sends page 3. Technically speaking the USB is "faster", but because of how the data is sent to the printer there is literally no difference in how long it takes to print 20 pages using USB or 20 pages using parallel.
Use whichever one you have the cables for. If you have some spare parallel cables around the house, use those. There's no sense in buying an expensive USB cable just for some imaginary gains.