The "octa core" isn't octa core. It's dual quad-core: Quad A15 for heavy lifting, plus quad A7 to save power when doing light work. At any time it will be using either the A15s or the A7s, but never uses more than 4 at a time.
Snapdragon 600 is quad Krait 300 - another arm-compatible architecture, not identical to the A15 but with a number of similarities.
Other components (e.g. graphics) will also make a difference to performance.
For all of the bloggers who wrote yesterday "Samsung weren't saying, but I reckon it was a snapdragon because it wasn't as blazingly fast as I'd expect the Octa to be", I'll personally be surprised if it actually makes a real world difference to the user (at least in performance - it will be interesting to see how power use compares). And I suspect a lot of those who wrote that type of stuff didn't understand what the "octa" actually is...