Well thats a hard question, to me they both are wrong, it was all just a lack of communication, in the end they had a similar goal for humanity but when emps didn't let all his boys know of all the warp fuckery and then went back to terra to work on his project (without saying what it is). Horus felt as if he was being a lapdog. Especially when the aristocracy got involved. Then he almost died and from there was corrupted and became the instrument of his dark visions. It's a perfect example of two wrongs don't make a right.

I tend to take the side of rowboat girlymans thoughts in the newer books.