The spin speed will most likely affect access time (check this in the drive specifications), but for sheer data transfer rates, the 6GB/s would be a much better deal. It really depends on how you use the drive how the cache will affect things - it is there to read data off the disk (and store data to be written to the disk) to avoid having to wait for the head and platter to get to the right point.
I recently upgraded a drive, and found the faster data transfer rate a huge boon...