Cache vs SATA Speed!

The speed of your hard drive depends on its mechanical performance, not on the amount of cache or on the speed of the SATA connection. Sustained drive speeds are limited by the speed of the access arm, the amount of data per track and the rotational speed of the platters. No hard drive even gets close the the capacity of 3Gbit/sec SATA, let alone 6GBit/sec.

Between those two drives you have, the rotational speeds are the same and the mechanical limits on performance are similar. (I cannot find on the WD website any good specs for seek times to compare, or exactly comparable average data transfer rates.) The one difference I do see is the cache size. 16 MB (on the blue) is the minimum you should have, and 32 MB is definitely better - that should show a small improvement in performance. Other than that, the Black line is designed and built a bit tougher (and maybe a bit faster performance) and warrantied for longer, so you might expect it to last longer in continuous use. For that reason, it might be the better choice for your OS drive. On the other hand, it is older that the new Blue unit, so maybe a significant part of its expected life is already used up.

Without knowing the actual model numbers it's hard to compare the performance. Newer generations of drive generally have have denser platters, which translates to higher transfer rates even if the rotational speed is the same.

Compared to those factors, cache size is basically insignificant. If a drive had too little cache it would perform poorly, but cache is so cheap that all the manufacturers provide enough of it to optimize the performance of the drive.