Actually eccentric reps are emphasizing the negative, so it's basically the same thing. So saying that negatives aren't the 'golden ticket' to size but slow eccentrics are, is kind of miselading.
But, getting back to your question, it is a fact that the eccentric portion of reps is what causes the most microtrauma to muscles and accounts for the most hypertrophic and strength responses. Not the concentric portion of the rep like many beleive and overemphasize. So, in my opinion and the opinion of many who know their shit, the most optimal way to gain muscle is to perform reps where the eccentric portion takes much longer, at least 2-3x as long as the concentric part. So throw your ego out the window, cause your poundage will undoubtedly drop initially, but your gains will be maximized. Over time, your muscles will acclimate to this added stress and your lifts will get back to where they were and even go beyond asw the muscle gets stronger.
A good rule of thumb is that the positive portion of the rep should be explosive and the negative portion should be between 3-6 seconds per rep. This increases total time under tension for not only each rep, but the overall set as well.