What are the most important specs on hdd?

[Truebrit]

I've just been looking at getting a 2TB drive to go with a 500GB SSD. As I understand it, the more rpm the better, the less ms the better, and the bigger the cache the better. If that is all correct, which is the most important?

 

[Truebrit]

It will just be a secondary drive to go along side an SSD. I'm just wondering what the important specs are. Like with Cameras, companies used to boast about megapixels, but megapixels are low down on the spec list for a good camera. I see some expensive drives that have lower cache and rpm than cheaper drives, yet I though higher was better for both of these?

What you'll be using it for is the first question to ask,yes.