The wrong question is being asked. It's not Leon or Ki. It's Ki or Shelvey, which makes it a no-brainer. Leon comes back and assumes his role. Ki is the far more versatile and quality player (perhaps out of the 3) and he shifts into Shelvey's position. Shelvey goes to the bench or stands where he can see better and learn.
We have a bit of a dead zone in midfield between Ki and Sig. Shift Ki in between Leon and Sig and we will have a very smooth, cohesive midfield unit.
IMO - now is the time to move Shelvey on. His attitude when on the field is not the greatest at times, his decision making leaves a tad to be desired too often. He has not really adapted to the Swansalona style of passing, still preferring the long ball if he has a choice ..... and I think Rogers recognized this, which is why he let him go. The key here is adaptation. Shelvey has talent, not very versatile and intelligent talent .... and that's the rub.