I think that full on racing, meaning balls to the wall I want to win attitude, is best served by clipless pedals as they will get you the ability to push and pull and squeeze the juice out of every pedal stroke. As somebody else also mentionned, you could potentially bail harder with clips due to the fact that the bike remains partly attached to your feet when you go OTB.
Now when I began racing DH in the 90s, I tought Palmer looked cool as hell so I took on the flats. Raced flats for a good number of years. Then I stopped racing/riding DH and started be clipped in again for trail riding and 4x racing/bmx racing. I've new come full circle as I have adopted flats again for most of my riding except BMX racing.
Flats are easier on my (previously operated) knees and I like sticking my foot out when turning! They also encourage a smoother pedal stroke without "cheating" like you would do on clipless.
Also, FiveTen have changed the game so much. I use to ride Vans in the 1990s and they were still are great, but the diff with modern flat pedal shoes is astounding.
Good luck and be sure to tell us what is your final choice.