Which Drive for TimeMachine Backup?

I just bought a MacBook Pro 2009 model and would like to have a dedicated external hard drive for performing TimeMachine backups.

I know there are probably great options out there for me to purchase, but I'd like to take one of the two that I currently have, format it to GUID and for Mac via disk utilities, and then use it for TM.

The two external drives drives I have are:

1.) WD5000AAV 500gb (about 2 years old, requires AC outlet)
2.) Toshiba Canvio 640gb (about 3 months old, no AC outlet required).

I just can't find specs for the actual drives inside of these external drives (drive speed, cache, etc.). Maybe it doesn't matter which? I just want whichever is fastest and most reliable. Thanks!