Both versions of the operating system currently in beta, which means that they are not fully optimized. It will probably be some changes when they appeared the final version later this year. In the meantime, if you are asked which operating system is working better, here is the answer.
Adrian Isen put a video on YouTube in which compares iPhone 6 with 6 Nexus phone. Both work on beta versions of operating systems. He did a speed test to see which device run faster.
