Quick answer. Neither is very useful. Max speed is about 115 thousand bits per second.A USB or Ethernet are better devices for external data transfers.
huh?
I think he's talking about serial as in synchronous bit transfers and parallel as in asynchronous, not RS-232 and LPT ports.
For example, USB, SATA, and most more recent transfer cable protocols are serial. parallel communications went the way of the dodo once they reached the technological glass roof that is intrinsic in high-speed parallel communications (for example, ATA-133). sure, they could probably research it and find ways of making parallel communication faster, but considering that none of the largest issues, such as signal jitter and crosstalk, are even relevant to serial data transfer it's a lot easier to just clock up the speed on a serial connection (such as SATA) without having the change the cabling to account for electronic limitations.