I get asked that a lot by people shopping at The Bicycle Shop. Honestly, it depends on you. People say clipless gives you more power, which is arguable and at best marginal.
It depends on your riding style for one. I use flats on my mtbs cause I like planting corners and having that kick push over things cause I’m tiny, with a tiny bike, over big logs. Clipless just got in the way. I tried clipless on my mtbs for a bit and it made me a mess. Stuff I could do easy on flats I’d just fall on my face with clip ins for no good reason really. But on the other hand, I love being clipped in on my cross bike and road bike. No good explanation.
The biggest thing is confidence. If you’re more confident knowing you can bale then you’ll be a better rider on flats. If you’re more confident you’re foot will be planted and secure to the pedal then you’ll be better clipping in. If you’ve got the confidence with the situation you’re in, then you’ll do better. A lot of mtbing is just believing in yourself that you can get through the situation.
You never know which side you fall on until you try. See if you can borrow a clipless set up from someone and give it a go.