If you are planning to overclock then I would say no it is not worth, I've read places that the 4790k overclocks poorly. If you are not planning to overclock however, I would say yes, extra 500 mhz can be important, especially for games that use one core only to the max (rome 2 total war for example, runs badly on an i5 3.7ghz) and the 4790k is the fastest cpu out there(turn off hyperthreading from the bios for games though), it's more future proof and it will make your rig last longer than buying something that is good enough for now.
About the boards, I am in no way a board expert, but I was reading the specs on the websites, the ASUS board mentions that it supports amd crossfire but makes no mention of NVIDIA dual VGA, GIGABYTE on the other hand states that it supports both amd and nvidia sli, it also says that it supports overclocked ram up to 2200, maybe you don't need that anyway but since you're picking, might as well go with the one giving you more options