I find Google Flights to be much better than Kayak, and so have the couple people I've talked to about the difference. What do you like more about Kayak?
Other than flexible dates and more search options, Kayak has direct search links to the airlines. Google flights (while having brilliant map search) asks you to call a travel agency and in many cases the price was not as accurate as Kayak
This is surprising to me because Kayak uses ITA Software [1] which is owned by Google. Perhaps Kayak has better relationships with the airlines than Google and cultivates more accurate pricing this way.