I could be wrong, but...
For any given technology parallel will be faster than serial. If we can send 10Mb/s down a single line, then we can send 20Mb/s down 2 lines. However...
Speeds these days are so fast that cross talk is a big issue. This is when the signal on a line is degraded by signals on near by lines.
Solution: Shield the line. Ultra SCSI and the new 80 pin ATA cables all have ground wires between each data line. So it would look like
+o+o+o+o+o+o+o+
where + is a grounded line, and o is a signal.
However, as we go faster, we get even more crosstalk, more refections of signals at the end.
Solution: Use a very high quality cable with low impenance, with ultra fast and sensitive electronics at each end to ensure the signal degredation, crosstalk and reflections are minimised.
This I believe is why we are going serial these days. The speed of this high speed serial is fast enough to not require parallel, and the price is high enough to not want to have 16 signal lines if we do not need to. Also, it is a whloe lot easier dealing with firewire, or some other thin serial cable than a bulky 80 strand ribbon cable.
So for the moment serial is comming back into vogue. For most people, serial may stay. Fiber optics allow fantastic fransfer rates and when we saturate the chanel, use a second wavelength down the same fiber. Some people (communication industries) will always require parallel. If you require a 100Tb/sec network from the Americas to Europe, you may well require multiple fibre optics as technology cannot pump that much data through a singe fiber. But for the rest of us, a single fiber optic chanel is faster than we can type, move a mouse, read from the HD, print, update the screen and so on.