Tested has some interesting results for the world, where it has tested the PS4s internal HDD against a Samsung-built SSD, and a hybrid 1TB drive. Check out the video below.
As you can see, the SSD isn't that much faster than the hybrid drive - which is much cheaper, and much bigger - but the stock drive is slow as hell. Something I noticed is that loading games on the PS4 seems to take forever, but I'm comparing this to my all-SSD PC. Maybe it's the PS4 OS that needs some work.
So, as you can see, it looks like a hybrid HDD is the way to go.