Of those two, the four-legged one will be more stable but a pain to take down and set up again if you often move the keyboard. The X-style one is a pretty good compromise. Personally, if I'm using one of those X-style stands for a long period in one place, I usually beef them up with a vertical wooden support (easily cut from some 2-by-1) at each end, which stabilises them and makes playing less of a struggle. But they're pretty good in general. The single-beam X-style ones are a bit of a dead loss.