My roommate says I might as well just get a non-SSD HD for storage. I was going to look for SSD deals and get a 2nd SSD instead. I have external HDs totalling about 4.5TB, but you know how slow those can be..
What would you get?
For storage of what?
Media: Speed is irrelevant other than for backup purposes, and for large amounts of storage your bottleneck is probably going to be the interface rather than the drive speed anyway. Get whatever is cheapest per GB. (USB 2.0 is painfully slow, if you have any of those left, I'd just chuck the enclosure and use them internally.)
OS, documents, programs: SSD, always.
Games: If you have enough games that storing them all on SSD is cost prohibitive, I'd probably do a 4TB WD Black drive with a dedicated 128GB SSD for caching.