i bought the viper x4 and a friend of mine bought the api grandslam i believe. i've climbed with both of them. the api compacts down a bit more than my x4 but the x4 seems to be more comfortable, spacious, and easier to climb. plus i don't like the idea of using the pins. it seems to easy to loose them when adjusting the chain. the summit is also built so that it won't squeak or make metal on metal noises. i think it's called the summit lock of something like that. the joints are locked together before they are welded so that the internal stuff doesn't move or something like that.