We'll use the MT6592 as an example. This is the octa-core that's in the Alcatel OneTouch Idol X+.
All 8 cores in this SoC are ARMv7. It's a bit like having twice the MotoG's guts going at it all at once. However, given the current state of apps, many can't use more than 2-4 cores. Some heavy benchmarks have used 5, and that was it. So as it is right now, it's nice to have but not much can use it. And ARMv7 cores are from the past, so while still nice, nothing cutting edge there.
Octa Exynos SoCs use a different approach with their big.LITTLE technology. They use low-power ARMv7 cores, sometimes ARMv53, and keep the ARMv57 (or ARMv15) stuff for the big work. This allows them to save battery life and get things moving for heavy tasks when needed.
2 different beasts for different tastes. The thing is, though, both will work good enough for *most* users.
Me? I like me some Snapdragon 800/801/805 stuff :D