Foot retention of some sort is more efficient. Clipless is better in that regard than straps worn with regular street shoes, partly because with clipless your foot is more secure and partly because actual bike shoes have stiff soles. But if you're riding around town, straps may be preferable because you can wear your normal shoes. Mountain bike shoes are not terrible to walk in (or cycling sandals, touring shoes, etc), but road bike shoes are pretty un-walkable. But lots of people use MTB shoes on road bikes, that's not a problem or anything.
Personally, I don't much like riding any bike with my feet not attached. I feel more secure knowing I can stop on the pedals at an intersection and my foot won't slip off.