The Exynos octacore is actually 2 quadcore processors : four high-performance ARM Cortex-A15 cores running at 1.60GHz, four low-power ARM Cortex-A7 cores with 1.20 GHz clock-speed as well as PowerVR SGX 544 graphics adapter.
However, the Exynos 5410, though having eight cores but the phone will use only four at a time.

The technology used is called big.LITTLE and Samsung has taken a licence for it from ARM. In actual use, Galaxy S IV will use A7 cores while handling light work. For example, if you are making a call, the phone is likely to be powered by A7 cores. But if you are playing a demanding 3D game, it will switch off A7 cores and use A15 cores. At no point of time, the phone will use all eight cores simultaneously.

Samsung claims that this technology provides users best balance of performance as well as battery life. An important detail to note is that A15 cores are very powerful and even if only four of them are used in Exynos 5410, it is enough to make the device one of the most powerful smartphones in the market.

I am only looking forward to the Snapdragon model mainly due to LTE being enabled on it

http://www.galaxy-s4.info/galaxy-s4/samsun...ussian-websites

^-- exynos = 12.5hrs video playback

http://blog.gsmarena.com/samsung-galaxy-s4...res-how-it-did/

^--- snapdragon = 10.5hrs video playback