As others have suggested; go with MTB pedals that are two sided(Time ATAC, Shimano SPD, Look Quartz), or, which I will say again; Eggbeaters!!

Because they're made primarily for a MTBs, they allow the rider to unclip as fast as possible in case of an emergency.

For example:
--When you have trouble going over roots and you don't have enough forward momentum and begin to loose your balance and start to fall and you QUICKLY unclip to keep yourself from hitting the roots with your arms and shoulders.--

That was me this morning on the trails.

With that said, I don't recommend 'Speedplays' for first time users. They are a good pedal, but they are a road pedal. Even though they have a good float, they're purpose is to hold on to you and be one with the bike. They're for long distance riding when you don't have to unclip so often, or when you want to put the hammer down. When you fall down with those, you'll still be clipped in when you're on the ground. Now, try unclipping when you're on the ground.

And finally. DO NOT BUY SINGLE SIDED PEDALS. Not for what you need them for, at least. It's better if you graduate onto those.