With the prices so close it would be better to get DDR3 1866 modules for future proofing even if its not going to make a difference. But make sure you get low latency modules like, 9-9-9-24, and go for a 2x4GB memory kit. It's really your choice what to get depending on your budget of course
You are right Werner, the price difference here is almost the same to purchase 2 * 8 GB DDR3 1866 MHz CL9 vs. stock 1600 MHz frequency and CL9 latency.
I think I will purchase the 1866 MHz dual memory kit and tighten the latencies at 1600 MHz, I am pretty sure the memory can't be overclocked anyway with this motherboard.
I think ASUS oughtn't to mention anything higher than 1600 MHz stock memory on their qualified vendor list for this specific motherboard.
Memories are indeed compatible, but their higher XMP settings can't be used here, am I right ?