I think if you look at reviews around the net you'll find the newer WD 640gb to be the faster of the two. One of the reviews I read had comments from WD about adding the extra cache (up to 32mb) didn't provide enough tangible benefits/speed to justify the cost. Something along those lines.
The Seagate will handle large files a little better, perhaps be faster if you have several files open at one time. For most uses, you probably wouldn't see any difference. One thing I've liked about the Seagates is that the SATA and power sockets are a bit better, more secure than the last couple WDs that I have bought. I had one WD socket break, rendering the hard drive into a piece of junk. It may have been a fluke, but it soured me and the only WD that I have bought since was a Raptor.
In the end, if I needed a lot of space, I would go for the WD hard drive. If space wasn't a major concern, I'd buy the Seagate.
Those are both solid working hard drives either one would be a great choice. I actualluy use both of them hard drives except my western digital caviar is a 500 gb and my segate barracuda's are also 500 gb, both are good.