For all practicality, both. A larger cache will speed up frequent and small file access times, while a higher rotational speed will decrease the latency of drive access overall.
In general, things like the time it takes to boot the machine, launch applications and read/write large amounts of data will be most impacted by the rotational speed of the drive. Real-world use like random access times and reading/writing small amounts of data will be most impacted by the cache size.