LOL, it is exactly $10 more, just like I expected.
The WD6401AALS has a longer warranty (5 years instead of 3). Google's study of their hard drives showed that about 6% of them died in the 4th and another 6% in the 5th year, so that's worth something IMO. Specifically, insuring a $75 product when the chances of failure are 12% is worth 12% of $75 i.e. $6.
The average transfer rate is 89.2 MBs, just like the WD6400AAKS.
The tech spec sheet on WD's site mentions a "dual processor" but I didn't see that helping in benchmarks.
The bigger cache will help in some scenarios, I guess. For years WD has claimed that 32MB of cache doesn't bring any advantage over 16MB, but I guess that's when they had 16MB and Seagate had 32MB. I suspect they added it just to level the playing field between their marketing people and Seagate's.