Hey you, yes you, do you like action? Do you like Japanese style Henshin heroes such as Super Sentai or Kamen Rider? That my friends is what these two shows have in common.
What they don't share however, is tone. Sadamitsu the Destroyer is a lighthearted, almost parody of Tokusatsu style shows, while Skull Man is a serious, dark take on similar concepts. Additionally, Skull Man came out six years later then Sadamitsu and was animated by studio Bones and not studio Deen, meaning it has far, far better animation.
But, if you enjoyed one, or tokusatsu shows in general (which is probably why you ended up here in the first place) you'll most likely enjoy the other so long as you go in with the right expectations.