Recently, on this site, I've been writing about pedals – both “flat” or “platform” pedals (BMX style), and “clipless” pedals (which, oddly enough, are the kind your shoe actually clips into).
Again, and as I stated previously, there is no “better” choice when it comes to clipless vs flat pedals. Both have advantages and disadvantages with more overlap then most riders are aware of (especially if that rider has never taken the time to learn to ride the other system).
So, we have MAINLY two different types of clipless pedals. The first has spring loaded bars that spread apart when under pressure from the cleat on the bottom of our shoe, and then the cleat snaps in between these bars. The bars then hold the cleat in place, and we're “clipped-in”. Some pedal companies that use this system are Time and Crank Brothers. Some benefits of this system...
The other type of system is essentially a Shimano SPD system. In my experience, I feel that they have more adjustability in spring tension and a different “feel” in terms of “float” (how far you can twist your foot before the cleat disengages with the pedal) and entry. Shimano also has different cleats: single release and multi-release as well as at least one wild-card cleat that was designed for a very limited type of pedal and is nearly impossible to use with any of their other pedals. I'm not exactly positive what Shimano's story is on their cleat/pedal recommendations, so all I'll say on the subject is do your research and try before you buy. But, obviously, they're a great set-up when you get it right. The SPD's can get finicky in a hurry in bad weather conditions.
Another great thing about the DH set-up is the large amount of rubber on the sole of the shoe compared to the XC's hard plastic.
So if your priority is power and efficiency, go with a light stiff shoe and a light, minimal, pedal. If you're riding in gnarlier conditions, you may want something that gives you as much control and peace of mind as possible – even when your not clipped in – such as the DH-type set-up.
And, stay away from the “clipless on one side and flat on the other side” pedals unless they live on your commuter or bar-bike. Fumbling around on the trail for the correct side of the pedal to clip into is inefficient and dangerous … so is riding the clip side with regular (non-clip) shoes. One or the other please – you won't see any tops riders riding these pedals!!!