Here's the Western Digital Web Page
for the drive.
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.