Well, they have quite different meanings to each other. "~다고 하던데" is a hear-say phrase. For instance, "가은이가 숙제가 많다고 하던데" can be translated into "I heard Ga-eun has lots of homework". You can definately end the sentence right there, but you should add another sentence after it. The nuance of the "~다고 하던데" makes the audience is a punch line after the phrase like you did. Thus, the exact translation of "가은이가 숙제가 많다고 하던데, 그래서 가은이는 못올것같아요" would be "I heard Ga-eun has lots of homework, so she probably would not make it". "~다고 해서" is more like "Even if" in English. For example, I can write a sentence with the phrase like "가은이가 숙제가 많다고해서 내가 생각하기엔 못 올것같지는 않아". It means "Even if Ga-eun has lots of homework, I don't think she would not make it". Hopefully, this helps you.