That really depends upon opinion more than anything else. Both cards have many people who like them. For typical usage over WiFi, the Killer 1535 will be faster and the Intel 8260 will be the most consistent. The Intel 8260 does have Bluetooth 4.2 and the Killer 1535 only has Bluetooth 4.1, though - so if you use Bluetooth and want to be more future-proofed, the Intel 8260 is the one to choose.
I have tried out both types of cards in the past, and have been bitten by the killer NICs. They seem to get most of their speed improvement by "optimization" of network data processing. This means that they can process data faster with less overhead as long as it of a type that their system has been optimized for. Anything that falls outside of those paths will take longer to process and can fall below what is needed to make it work.
Either card will still perform better than a default style of card with the same overall specifications with a generic brand like Broadcom (which many OEM love due to volume pricing).
Personally, the Intel NIC would be my choice given these facts.