Tried both, each is a tool for the specific job. XC riding has to be clipped as you use the whole leg stroke. Trail riding where it can get rooty/rocky/techy = flats, unless you are skilled and confident enough to ride it. I've had some stupid offs wearing clips....even by rolling over a tree stump as I wasn't paying attention. But then I rode the Dragons Back at CYB clipped in and had no problems, although I think I might have fallen over in the car park....
That was a joke by the way, about the car park, no honest, it was!
I tried mallets as well, but didn't like them much. Now I just have Shimano AM41's, they are super grippy and wipe clean!