Both the main CPU manufacturers are, by now, offering products with different trajectories. While Intel launches a new CPU based on a new microarchitecture and socket, AMD have not launched a new platform for years, which is frustrating to the enthusiasts.
However, the results we found on this review are the same we saw on the A8-7670K CPU Review and on the A10-6800K vs. Core i3-4150 CPU Review: the Core i3-6100 has a little higher performance on tests that depends exclusively of the CPU performance, but the A10-7870K has a faster GPU than its competitor.
So, the choice for the “best” processor will depend on how you plan to use it: if you do not intend to use your computer to play games, or if you will use a dedicated video card and disable the integrated video, the Core i3-6100 is a best option than the A10-7870K. But if you want to build a budget gaming computer using only the integrated video (or if you want to use a low-end AMD video card taking advantage of the AMD Dual Graphics Technology, were the processing power of the video card adds to the integrated GPU), the A10-7870K is a better deal.