I buy the same thing for all my kids and NOT $20 each either.
Holy Moly. I just get something small. Book, puzzle, stuffy, etc. Often I include an ornament I made or, like this year, a reindeer candycane. And they go home. We don't open here.
I used to do a Polly exchange and would get parents who spent hardly anything and others who got huge items. Cut that after one time.
To answer the question though, I would NEVER NEVER EVER get something more special for one child over another simply because I 'favored' him/her over another. Sorry but that is just wrong. If that child isn't a 'fav' it isn't really his fault. Business issues or behavior issues almost always stem from the parents so why would I ever buy something nicer for one child over another? If you can't treat your children equal, then don't get anything.