After nearly decade on SPD's I switched to flat pedals once my first kid starting riding onboard with me (on a mac ride). Kid 2 is on the mac ride now and I'm still on flats. I ended up getting the base model freeriders which have some nice flex in their soles, unfortunately my local shop didn't have these CB shoes. I'll have to try these ones out when my freeriders are trashed.
This weekend I did a ride on SPD's (after two full years on flats) and I found: I missed the ability to move/adjust my feet a little bit as required and that I felt a little less beat up using my SPD's. Maybe my body is more relaxed on SPD's? Efficiency?