How about "swears by both".
SPD has the debris shedding design for when you need it, but sacrifices platform area. The SL has platform but sacrifices walk and debris shedding. So, I put the one fit for purpose on the right bikes I use them for. Cross/gravel, spd. I have to get off and run in mud that requires toe cleats. So yeah, spd there. Road? I don't get off in the mud or really walk miles to the cafe', so -SL.
You have to consider in the mixed road/walking a lot scenario how much you walk. If it's a cafe' or gas station stop on a road ride, go -SL. If you're a commuter or something, I'd go spd. The walking ratio matters. If you ride and then stop at a bunch of brewery or food stops on a little tour, spd. You walk enough it would be nice I guess.
TT bike gets Speedplays for the stack/aero advantage, but that's a super specialized race application thing for me. I'd prefer the -SL's from a "no lube or maint." standpoint over the Speedplays.