Well, I guess it depends on how confident you are on the trail. I have Speedplay Xs on my road bike and I love them. However, I have dual sided spds on my mtb and my commuter. I don't have a problem switching back and forth between the pedal systems, but I do have the spds set on the easiest setting to get out of. The reason I have the dual sided pedals is that there are a lot of places here that i just don't have the confidence to ride clipped in while mt biking. I can get my foot out easy enough, but having the option of using the flat pedals makes me a bit more confident. There is one trail I ride that is very muddy and I am usually always clipped in on this trail. I haven't had any trouble with mud in the cleats.
I would love to get Frogs, because the spds definitely are not nice to my knees, but it also would require another pair of shoes in addition to the fear factor.