Sanchez going on to become a better player than Aguero in the future is subject to so many factors. I don't rule out Sanchez going on to have a better career than Aguero. Sanchez is a talented player, and he might go on to develop even more at Barca. Still, i believe Aguero is the more talented player, and if Sanchez goes on to have a better career, then it will most probably be due to Aguero's failings more than anything else. He might get a serious injury, he might make a bad career choice; like moving to Juve