In 20 tests of flights all around the world and at different times of year, Kayak found cheaper flights more often — sometimes by only $2, other times by greater amounts.
Only three times in those tests did Google Flights find the cheaper flight.
This wasn't a scientific test, and in many situations, Kayak and Google Flights surfaced identical results. But there were so many instances where Kayak beat Google Flights on price that I have to assume that it's the superior service.
Now, not everyone is always looking for the cheapest flight possible. I'm usually trying to find a balance between cheap and convenient, but I typically err on the side of what will cost less. Knowing that I'm seeing the very cheapest option available on Kayak is comforting, even if I end up picking something slightly more expensive.
Google Flights offer plenty of cool perks, and it has a much cleaner, easier-to-use interface. I much prefer the experience of searching for flights on Google Flights over Kayak.
But it's hard to ignore that Kayak finds cheaper options. So while Google Flights is an excellent service on its own — and I'm probably going to start using it more often, at least for comparison's sake — Kayak remains the winner.