I'm a fan of clipless for most XC/AM trail riding. Years ago I used a Speedplay Frog pedal + Sidi shoe combination. The Frogs have an enormous amount of float -- not the best for beginners IMO but supposedly good for those with knee problems. Now I ride Crank Brothers Candy 2 pedals + Mavic shoes. The Candys have a medium sized platform, a simple but effective spring design, and clear mud well, and the Mavics have a super stiff sole for power transfer. Like most good pedal/cleat combinations, the Crank Brothers cleats release at a reasonable tension and angle, and after you get used to them, you can pop out quickly if needed for more technical terrain or fast corners. If you want more platform, try the Crank Bros Mallet; if you want less platform, try their Egg Beaters.