Which one should i buy? Skarner or Heimer?

Depends. If you want to understand the frustration of having a champion and NEVER being able to play that champion due to bans OP, go skarner.

If you want to make the enemy laner hate your miserable guts go heimer and spam laugh behind your turrets.

make sure you dont pick heimer before your enemy picks his because you will have a brutal time laning. even when you're fed as heimer he doesnt really offer anything to the team late game. besides an ulti hourglass which is easily countered past level 13