SPD vs. SPD-SL: Which unclips most easily?
In my experience, Time/Mavic Iclic.

This should, however, not be the reason for decision. Both are easy enough to unclip if you remember to twist your foot instead of just lifting it and neither will unclip if you forget it. Don't believe the people who claim falling is part of learning, just practice leaning against a wall and unclip a couple of hundreds times or until it feels natural.

What you should consider is:


SPD-R and road systems in general are very uncomfortable to walk in. They are compatible with some very nice and lightweight road shoes, work very well with shoe covers and have some roadie credibility 
SPD is walkable (but uncomfortable, has loud crunching sound on pavement/rocks and destroys floors), compatible with MTB shoes including waterproof and insulated ones and has credibility among grumpy stackoverflowers. There are some SPD shoes that look like road shoes but are in general heavier and less stiff.