They're both variable, but the base PS4 runs a decent bit lower on average, and has dips into the 20s.
Oh... but then why not lock the PS4Pro version at 30? It seems like a strange decision to unlock the frame rate, though it might only be me that doesn't like frames running from 30-55 and preferring 30fps locked instead.
Sounds like the PC Version is the way to go then. I'll still wait for what the Switch Version will offer though, I could live with a solid 30fps lock, even with reduced visual effects. -But hearing of the vanilla PS4 not holding 30fps doesn't sound too promising :/