Both haven't fought great opposition but Canelo has fought better fighters. Plus you can't really complain with Canelo seeing as how he is just 21 years old.
Canelo's competition is better than Jr's. Not only that, but Canelo beats his opponents the way he's suppose to, while Jr at times needed gift decisions to get the win.
Are you kidding me? Julio Jr. fought Irish John Duddy! hahahahaha! Maybe he'll step to David Lemeiux?
Both have fought a huge number of B level fighters. Personally I think Chavez has fought the single best fighter (Molina) though you could make a very solid case for Molina winning both. For me all of Chavez's best wins have been against smaller men (Vazquez, Cotto, Hatton, Gomez) or shot fighters (Cintron, Rhodes) whilst Chavez has needed nice judges (Molina, Zbik). Really not much at all between them in all honesty.