I set x to grab originally because I use z-jump. (Makes pivot upsmash, egg slide toss, and many other techs much simpler) But after getting used to it, X is overall better as grab imo. The reason being that it's just as crucial as attacks and specials, with the same general time in useage while grounded.
So it gives you a more accessible option select out of dash or landing between your attacks, specials, or grab all with one thumb. Making reactive pivot grabs overall easier since you don't need to prepare both your index finger AND thumb to do the input, just your thumb with a minor alteration.
I personally have been trying out the Pro controller, and unfortunately I found myself liking it way too much. Much more customizable with much more responsive and intuitive controls.
(Though the c-stick placement makes for poor usage as your primary aerial input imo.)
I personally believe the best control scheme for pro involves setting Y to specials. This provides the ease of short hop aerials on a gamecube controller to both attacks AND specials since the input is a mere slide.
The B button is set to grabs, similar reasoning as why we do it on gamecube controllers with X.
A shoulder button set to jump is also ideal for z-jump reasoning.
And due to the poorly placed c-stick, I suggest keeping them set to smash, turning tap jump off, and getting used to using X/A for your aerial inputs. the This also lets you make the most use out of X/Y combination to do short hop specials. Something I did a LOT on 3DS but could never do on the gamecube controller. I was able to utilize short hop rising clay pigeons with duck hunt to catch landings or play as an anti-air, but it just wasn't feasible on gamecube. Not fluidly, anyway.
So in closing this is my recommended pro controller set-up:
A: Attack
B: Grab
X: Jump
Y: Special
Shoulder button: Jump (whichever's most convient/comfortable. It will be a rarely used input)
Tap Jump: Off
C-Stick: Smashes
Little things:
Rising/Timed dairs are a little hard with X/A, so I use the shoulder button to jump and c-stick down while the control stick DIs. For dairs that need precise timing, this is much more ideal.