I find myself liking Knockdown better than Bottomsup, being able to pick any row vs just the bottom is handy. Experiment with both and see which one you like more!
On my Fighters, I sometimes use all three. I will put a point into each just to have them, then start focusing on Knockdown/Bottomsup to get them at a reasonable level, then add a few more into Powerstrike depending on what objectives I am after and/or have done. They all are pretty useful, and you are not limited to picking just ones from your class.