When it comes to using x/y, I personally prefer Y. Ken's argument is based off of the assumption that it is quicker/easier to swipe your thumb from right to left (x->a) than from up to down (y->a) (an assumption which is probably true for most). But I feel that most of the argument for x vs y is just based on player preference. If somebody is playing a spacie, I would personally recommend to use Y, because of its physically closer location to b (thus making it easier/possible to multishine or waveshine). Aside from this, I use tap jump for falco's shine bair/dair. When it comes to JC's, I would recommend using the control stick for Usmash, and the x/y buttons for grabs. The reasoning behind using x/y vs control stick is that x/y allow for digital inputs as opposed to the more variable axes on the control stick. And it does, as you point out, allow for more consistent air movement.