WD1002FAEX is the 64 cache and
WD1001FALS is the well known 32 cache version.
The most interesting thing is that Western Digital does not list WD1002FAEX on their official website at all. All they advertise is the 32MB cache 1 tb drive. I wonder how it is on sale on Newegg when WD does not even list it as one of their current products.
The performance difference in the graph you found agrees with the specification sheets for the drives - the FAEX has 126MB/sec sustained transfer rate vs. 106MB/sec for the FALS. This won't have anything to do with the cache per se, but is more likely the result of higher platter density.
So the long and short of it is - yeah, probably it's worth the extra $20 to get the faster drive.