Booking.com makes it easy to find flights. If you are flying to a destination with served by multiple airlines, Booking does a good job at showing the different flight options based on price, time, and carrier. Their website is a little easier to browse than Priceline, even though you will pay the same amount at either website.
Another reason you might prefer looking for flights on Booking.com is that they have a trend window in the upper-left corner. This window will tell you if you should buy now or wait, based on their prediction of when fare prices will increase. Since the flight search results are powered by Kayak, both the Booking.com and Kayak search results look identical except for website colors and the website name.
Unlike Priceline where you can book flights through the portal, Booking.com takes you directly to the airline website to complete the reservation process.