I have a bike with the SPD-SL (road SPD) pedals, and run standard SPD on my others. For me, the SPD-SL is much more difficult to get clipped in. Once I am clipped it it does seem to be a bit more secure, but it's also a lot more difficult to disengage from the pedals, even with the tension backed all the way off.

For the standard SPD pedals, I started with the combo pedals with SPD on one side and platform on the other. It was nice for the rare times I wanted to ride in street shoes, but after many thousands of miles when I wore the pedals out and needed some replacements I decided that I rode in street shoes so rarely that I've give the dual-sided SPD mountain bike pedals a try. Glad I did. The difference is night and day as far as ease to clip-in. Before I would inevitably start pedaling and find myself on the platform side, then spend the next several revolutions trying to get the pedal to flip over to the clip side. Now with the MTB pedals I just put my foot down and *snick*.

Between walkability of the two, my SPD shoes beats my SPD-SL shoes hands-down. My SPD shoes are stiff sole so they certainly don't feel like I'm wearing street shoes, but they only point my toes to the sky about half of what my SPD-SL shoes do.