1 SSD and 2x1 TB WD Green HDD's

Hey.. I'm thinking about getting a SSD for my system.. 
It will most likely be a 120 GB..
Now my question is, what would be the best config? Have the SSD cache my two 1TB WD Green (6 years old), or just use it as an OS drive?

OS drive.

Insanely fast boot times, you can use your others for storage and games.