Is catching a level 20 haunter stronger than catching a level 22 ghastly and evolving it?

No your pokemon will evolve at the same time as a normal haunter. I know the situation, same thing happend with my pidgeot. so i know that a pidgy evolves at lvl 18. but i saw a pidgeotto at lvl 15. so i caught the pidgeotto and it evolved at lvl 36 like regular. nothing special.