For $10 extra it's a no brainer. I'll aim for DDR4-3000 myself. Above that, price climbs up alot for close to no gain, below that it seems skylake can get undersaturated in scenarios. Plus, it's likely you'll be able to swap your cpu in that motherboard for at least one or two next generations later (Kaby Lake, Cannonlake). It's expectable next gens will be able to utilize faster ram better.