HelloCycletography:
I've ridden both Shimano SPD (two bolt) and Shimano SPD-SL (three bolt). I initially shifted to the SPD-SL from the SPD when I started riding long miles exclusively on road bikes. I felt the SPD-SL was more comfortable (less "hot foot" - that pain under the ball of my feet right at the cleat) after 80+ miles, especially on hot summer rides. If I was still riding road bikes I'd still be on SPD-SL's. However, I've now shifted almost completely to dirt (rigid mountain and gravel bikes) and I much prefer the SPD for the ease of walking. A lot of my rides are exploratory and I find that I have to "hike-a-bike" for miles to connect drainages and two-track jeep trails more than I plan!
I don't use single-sided or platform/SPD pedals as I'm always wearing my cleated shoes simply because they walk so well I don't take anything else on bikepacking trips. I'm currently riding SRM X-Power pedals because I wanted power measurement that I could switch between bikes. I'm really happy with those pedals for riding and power measurement. As far as I know they are one of the only (if not the only) SPD pedal based power meter. Ridiculously expensive unless you really want an SPD power measuring pedal.
Howard