That would probably depend on what you're willing to spend on data storage.
I personally can't afford to spend too much on anything, so I opted for the WD 500GB AAKS's when they finally dropped under $100. The 640 is currently $110 on newegg, so the cost per GB is slightly lower, if you can spend more than $100.
If cost isn't a concern, the 640GB drives IIRC are quite fast due to the larger 320GB platters. That speed, while not as fast as a Raptor 150GB, does appear to nip at its heels from benchmarks I've seen. That could be very helpful when using it as a Photoshop scratch disk (although that's a massive disk to use for such a purpose.)
There are cheaper options, but if you can afford it, the pro's are clearly there.
Edit:
Silly me, I divided incorectly, 640 has a LOWER cost per GB, not higher.