NBAdraft.net is run by some unknown individual with no connections to NBA GMs, colleges, or insiders of note. It could seriously be a kid running a very nice website in a garage. They claim they have "scouts," but they're unnamed and who knows how qualified they actually are. DraftExpress actually has connections to the NBA scene and has
named scouts, including president Jonathan Givony, that are reputable. Their analysis of players and in depth articles are clearly more well-informed. It's possible that NBADraft.net gets lucky and picks the mock draft better simply based on them using other sites and articles, while DraftExpress relies more on first hand knowledge. But I'd definitely put more stock in DraftExpress. Chad Ford also seems to at least have some NBA connections and is likely more reliable than NBADraft.net.