I tried SPD pedals on my LHT and my custom Gunnar, my knees hate them. I wound up with Speedplay Frogs (a mtb pedal) that I use for road riding - love them! I do mountain bike but currently use a good pair of BMX pedals for that bike.

That being said, you don't HAVE to clip in. Nothing wrong with a good pair of BMX pedals, and while they will scar your legs, you do learn how to stop without pedal slap. I prefer BMX over other platform pedals because my feet stay on the pedals much better on them - I won't use toe cages.

Do you have a bike store with a fitter you trust? They might let you check out different pedals on their trainer to see what you think...