which is better? specifically for an Intel Core i7-4790K
If you're getting the 4790K, then the Z97 is the way to go.
Like other said, newer processors will work more reliably on the Z97 chipset, but another difference is that Z97 supports M.2
Devils canyon processors use Z97 technology, also, it may include M.2, ultra m.2, sata express, better audio, and more compatibility for newer technology. i would get z97 by far
Keep in mind that the majority of the Intel Z87 chipset based mobos requiring a BIOS update to officially support "Haswell Refresh" processors like one mentioned.
It's about compatibility, Z87 needs updates for Refresh and Z97 doesn't.