Aguero has done nothing outside of the PL; Rooney was a mainstay in numerous massive occasions in the CL - they are not on the same plane of existence INT football or not.
Aguero has been a massive let down to this point in his career - he was expected to go on to become an all-time great well before he got to England and he has fallen well short of that because of the poor body of work he has outside of domestic football.
Rooney shone in an international tournament until injury ended it for him, also, Aguero doesn't even have that under his belt.
He's a very good player, but the second-coming of Romario that he was supposed to go on and be, he is way short of.
I don't think this is entirely fair or accurate. "Nothing outside of the PL" obviously deliberately downplays that he's got the best goals per minute scoring ratio in Premier League history, which is not a minor achievement. But it's also just wrong. He was also excellent at Atletico in La Liga (causing City to shell out for him in the first place), and he has a CL scoring record of 36 in 65 games, not quite at the level of his PL record, but still comfortably better than a goal every other game. He's not had the "massive occasions" in knockout CL football primarily because City's record in the CL is relatively poor. Rooney has goals in CL semi-finals and a losing final, which Aguero doesn't have, but he also has a scoring record that is far inferior to Aguero of 30 in 85, or one every three games, although obviously he did play in a slightly deeper role at times.
With regards to international football, Rooney had the benefit of being the absolute main man for England. It's fair to say England didn't quite have the same attacking options as Argentina during that period, so it's not too much of a surprise that Rooney was more important to England than Aguero was for Argentina. Rooney was excellent for England at Euro 2004, where we were eliminated in the quarter finals, but then did nothing of note at any subsequent tournament. Sure he's England's highest goalscorer (until Kane catches him), but it didn't help England achieve anything beyond a tournament quarter-final.
I'm not trying to say that Rooney wasn't a great player, he obviously was. But trying to argue that Aguero is "not on the same plane" as Rooney is just clearly not accurate, bordering on ludicrous. They're both great players with excellent records and a lot of medals.