You probably won’t notice any difference between a 32MB and 64MB cache. It’s unlikely you would even see much of a difference between 16MB and 64MB. We are really not talking about a lot of memory, even at 64MB. The drive cache just holds I/O operations, so that they can be queued. When you look at benchmarks, cache size seems to play no real role in performance. That said, if the price differential is nominal, I would still probably get the larger cache just because.
Tom’s Hardware did some benchmarks with HDDs that were identical other than cache size. The results showed that:
“...we found that there is hardly any difference between two drives that only differ in their cache sizes: 16 MB cache has no significant advantage over 8 MB across our benchmark suite, and this applies both to Serial ATA and to UltraATA drives. We would have expected that at least the SATA drives would show some degree of benefit, but in the case of the 7200.10 family, 16 MB cache is a waste of money if you have a cheaper 8 MB alternative. At the same time, 16 MB cache doesn't hurt either if the price is about the same.”