How is "One for All" stronger than "All for One"?
Because All for One is adding powers on top of one another, while One for All is multiplicating it's strength. 

All for One is using the quirks of other users, I think he isn't the kind of person to go and master each and every one of those quirks. 

On the other hand, every users of One for All have been training the same quirk again and again, mastering it, and then passing their knowledge to their successors.

I don't think the battle All for One vs One for All was a pure contest of strength, but also of mastery and technique. You can clearly see that when it comes to pure strength, All for One is stronger. But when it comes to ability, All Might was better. 

What made All-Might victorious was not his strength, but his ability to think ahead, and his near perfect mastery of One for All.