It is like comparing a crow with a peacock regarding beauty. Usman is a genuine leg spinner with all the varieties in his armory. You have to label Shadab as a leg spinner otherwise it will be not possible for anyone seeing him first to identify him as a leg spinner. Usman bowls in right length consistently while Shadab is a spray gun. Even Tendulkar was a much better leg spinner than Shadab. But but Shadab will be picked for another decade before the dumb selectors will be able to identify these factors.
Shadab is a good bowling but his bowling form has declined severely.
He was bowling extremely well a few years ago, it's just declined since he picked up the bat.
Usman is yet to face players who can read spin at the international level. Compare his domestic T20 stats and his international stats, and you see that when he's being read, he's not as attacking as he seems at the international level.
Shadab is a leg-spinner with an off-spinner's action, short runup, focuses on firing it in to create dots. Every time Shadab tosses it up, he takes a beating, and so his bowling style is more about bowling it on a decent length and getting some spin. He has 50 wickets to his name, so I doubt he will be replaced entirely.
The best case scenario is to play both of them, but then Imad misses out.
But Shadab isn't as bad as some people are making him out to be. He's just 22 as well, so keep that in mind as well. He's been identified for the future, he peaked at age 20 or so, and he needs to just bowl consistently now. If he doesn't take wickets, he needs to keep a good ER.