Done it both ways. SPDs get slight edge over running shoes for touring as they keep my foot anchored in one position and are a tiny bit more efficient for spinning. The advantages are largely offset for me by the "fiddle" factor, and having to deal with a pair of off bike shoes. I've taken to clipping a pair of Crocs to my bags when I tour with spd.
I am old and move slow, 10-12 mph. Thus have no real need for the efficiency offered by clipless. Probably explains my take or leave it attitude.