As far as I can tell from the specifications pages, they're identical, except that the WD2002FAEX is 6 Gb/s SATA, and the WD2001FASS is 3 Gb/s SATA. But for some reason they're the same price everywhere I look. Here's the Newegg links for example:
The faster connection itself makes no difference to the performance of a hard drive because even the slower connection is much, much faster than the data coming off the platters. It's like a bicycle on a road - the cyclist isn't going to get there any faster whether the speed limit is 60km/H or 120km/H.
Since there's no performance benefit to the drive with the faster connection, there's no reason that you should expect to pay more for it.