Latest reports suggest Samsung will use Snapdragon 600 chips in 70% of the first Galaxy S4 batch. This was already confirmed by the leaked list showing the availability of the two Galaxy S4 versions around the globe.
In case you need a refresh, the Samsung Galaxy S4 will be sold in two flavors - the Exynos 5 Octa-powered I9500 and the Snapdragon 600-powered I9505. The "octa" units are reserved for Asia and Africa and few European countries that don't have wide LTE availability just yet.
According to the reports coming from Samsung's homeland, the company is having issues with the Exynos 5 Octa production and there was no chance for it to meet the tremendous demand of the flagship. So the Snapdragon 600 switch was necessary, and the reach of the I9505 model was expanded beyond the US borders to most of the developed markets.
We already met the Snapdragon 600-powered Galaxy S4 and it turned out to be the fastest phone currently available, so no one should worry about the performance of the device. It's a beast that easily outruns the competition. And we are yet to see how those four 1.6GHz Cortex-A15 cores within the Exynos 5 Octa chipset are doing compared to the four 1.9GHz Krait 300.
Samsung has already announced the Exynos 5 Octa chipset will enter mass production in Q2 this year, which probably suggest the octa-core Galaxy S4 will get wider availability in June or July.