Yousuf Khan wrote:

One choice of drives is a 5900 RPM 256MB cache. The other is a 7200 RPM
^^^^^^^^___ 5400 RPM
64MB cache.

Depends on how you use the drive. If you repeatedly access the same
file or same data then a larger cache would be faster (as long as the
data size doesn't excessively exceed the cache size). The OS has its
own buffers. Applications will also have their own buffers. If you
typically open files, close them, and don't revisit them then a shorter
seek (faster RPM) might be faster. We don't know how you will be using
the drive(s), if it will be just a data disk or an OS disk, etc.

The above is just a generic assumption. You don't mention brands and
models of the drives you intend to get. What might look like a slower
drive might perform better than what might look like a faster drive.
Specifications are handy but they are not absolute. Look around for
drive benchmarks that log the results for the two drives in which you
are interested. Benchmarks may indicate which drive is better but they
only compare drives based on those benchmarks, not real use.

You also don't mean the capacities of the two drives. If capacity isn't
an issue, you might want to go to an SSD for the best performance.

What's the price difference between the two unidentified disks? If one
drive is 0.17% faster than the other, would you notice its performance
difference (since the objective of your computer is not to run
benchmarks but use it) and would it be worth the price difference?