There's also SLR Magic. They're making some fantastic cinema primes.
A7S II would be your best bet out of the two of those cameras though an even better option would be an FS5. The FS5 is now as cheap as an A9 and has many benefits over an A7S II. It is still rather expensive as a B-camera so I would honestly wait for the inevitable release of the A7S III because the A7S II is now over two years old.