To me, I honestly think ghost types are better. Here are the reasons why:
- Ghost types have 2 more immunities than bug (bug has none)
- Bug may have 1 extra super-effective than ghost, but bug is not very effective against 7 types, where ghost is only non-effective against 1. Therefore, ghost can deal much more damage to a wide variety of types.
- One of ghosts types best Pokemon is gengar and he's great in sp. attack and speed. Scizor is one of bug's best Pokemon and he's excellent in attack and defense. They both have a base total of 500 so anyone can work. This point is pretty open minded.
I think that both types are great, but ghost has that extra push that bug doesn't have. So, I think ghost is better by about 10%.
It's your choice!
Hope I helped! :)