I went from flats to half-toe clips, which had no noticeable impact. I changed back to flats after an accident where the toe clip was upside down and snagged on something. Plus I found it fiddly to clip-in/hook-on at the lights.
So back to flats... and the differences became clear. You stay on the pedal a lot better - I noticed that a bad gear change or a bump in the path was much more likely to throw the foot off the pedal.
Later, I scored a free used pair of 3-hole road shoes, and bought the cheapest Look pedals on special. These pedals have no tension adjust, they're hard-set at a minimum tension. As such its really easy to get out of them. I've never got "stuck" in pedals while stopped, and the one crash on my MTB with these pedals, the foot just popped right out without damage.
For convenience my MTB (wet weather bike) is back on flat pedals, and its really noticeable how easy the foot slips off flats.
I have never had SPD or MTB cleats, but I have been stranded a couple times mostly to flats. You simply cannot walk in three-hole cleats, and walking along the road in your socks is particularly unpleasant. There are cleat covers but they're protection for the cleats, still not particularly walkable. I've either made sure there are shoes at the other end for me, or taken some light shoes in my pockets.
Back to your question - the shoe has to fit comfortably for all-day comfort. I'd certainly explore the MTB style pedals for convenience, if I was buying new. Ones with elastic will self adjust to swelling feet, and velcro can be adjusted on the bike. Laces mean you have to stop to adjust.