If you had to choose between the two, who would you pick and why? (This is a sequel to my original post asking if Casey or Splinter would be more optimal)

Raph is king right now. If you haven't seen any of the Raph dodge special lockdowns that Crunan recently shared, you can take a look. My latest run with JJ in 2P has them, some of the recent 1P Raph runs probably will too.

Even without the special cheese, Raph has good damage and high versatility - he's at least average in all areas that really matter.

Splinter has slightly higher single target damage because of the dash special and 2 damage dive kicks, but he'll lag behind Raph in all other areas.