It really depends how much walking you are going to do. If it's any appreciable amount then it's a no-brainer, spd's. If you are just riding then I find spd-sl's a bit more efficient, you do notice it if you've been running spd's on a bike for a while then swap over. But I think that's probably more down to the shoes than the pedal. Obviously spd shoes tends to be more flexy so you can walk in them and also compress more than road shoes. There are some part carbon soled spd shoes though which should improve things but I haven't tried them. Some also find mtb shoes more comfy than road shoes.
spd's are easier at lights and junctions as you've two sides that you can put your foot on and click in to but you soon get used to spd-sl's.
If I had to pick just one that just works whatever I'm doing then it would be spd's. Of course you could get both if you can afford it and you think it would be worthwhile as it's trivial to swap them over.