Flat pedals for heterosexuals. Or something.
Seriously, use the one you like. If either is better, it’s SPDs but it’s not a big enough difference to outweigh personal preference, unless you’re at the pointy end of racing.
Still, there are times when flats can be a whole lot better and these times do come up more regularily if you’re a) doing a typical UK slop-filled enduro and b) not brilliant at it. There’s been races/stages like sooks pipe at kinlochleven or not for poofs at inners where I was seriously glad to be on flats
Oh and of course try both, properly, don’t just do that nonsense thing people do where you spend 5 years on one pedal, do a single ride on the other and say “I don’t like it”.