As memory prices have fallen into the "dirt cheap" range, drive manufacturers have realized that they can increase the size of their buffers at very little cost. Certainly nothing is lost in doing this; extra cache won't hurt performance; but neither does it greatly improve it. As a result, if interface transfer rate is the "reigning champion" of overrated performance specifications, then cache size is probably the "prime contender". Some people seem to think a 2 MB buffer makes a drive four times as fast as one with a 512 KB buffer! In fact, you'd be hard pressed to find even a 4% difference between them in most cases, all else being equal. Not surprisingly, both external transfer rate and cache size are overrated for the same reason: they apply to only a small percentage of transfers.
The cache size specification is of course a function of the drive's cache characteristics. Unfortunately, manufacturers rarely talk about any characteristics other than the cache's size.
spinning rust HDD performance is a complex thing (lately, determined more often than not by firmware optimizations) and cache size is not a silver bullet by any means.
Rotational speed is a much better predictor of drive performance in my experience: 5,400 rpm is slower than 7,200 rpm, which is slower than 10,000 rpm, which is slower than solid state. Even then there are (rare) exceptions.
That said, at such a low price difference, probably worth it.