I rode clipless for years, but didn't prefer to use them when commuting, and now I ride on flat pedals 99% of the time. I do like to be able to shift positions sometimes, but more than anything, I like being able to wear different shoes when riding, especially when it's cold and I can just wear warm boots. My fitness level isn't so extreme that I notice the marginal gains of clipless pedals anyway.
Cycling
I prefer being clipped in as it gives me more stability (as long as the bike fit is good). I live in a very hilly area where I'm going downhill 30-40% of my ride. being clipped in gives me the security of bombing down those hills.
I have a pair of single-sided SPD pedals, like these. They allow me to use my clipless shoes if I want, or flip the pedal over and use any shoes. Main drawback is that you have to fumble sometimes to get to the right side of the pedal you want.
I use clipless when riding recreationally, and flats when I’m commuting. I strongly prefer the feel of clipless in all aspects. The only reason I ride flats is so that I can wear regular shoes (when I’m, say, commuting to the office).
Clipless is only better for longer rides if your position/setup is good. Locking yourself into a bad position will probably be much worse than the gained efficiency from clipless over flat pedals, though whether it is noticed by the end of the ride or the end of the week is the hard part.