From what I remember in the Kaby Lake reviews earlier in the year, the Kaby Lake processors don't have much over the Sky Lake processors, speed wise. However, the Kaby Lake does come with some new capabilities built into the supporting motherboard chipsets. If you can wait for Sierra 10.12.6 to be released, any day now, there will be support for the Kaby Lake chipset, and we will have an update to MultiBeast to support 10.12.6 drivers/kexts, etc.
So, given this time and date, it's a toss up. However, you might as well go with a Z270/Kaby Lake system. BTW, when 10.12.6 is released, the tonymacx86 Buyer's Guide will be updated with recommended 200 series motherboards which will have drivers/kexts supported in the MultiBeast update.