It's close but as Agassi got older it almost seemed like he hit the ball harder. It helped him compete at the top of the game with guys like Federer at age 35. As Federer hits his 30s his aggressiveness and ball speed has extremely dissipated. However I will say that it seemed like something was clicking for him at this years US Open and he’s trying to go back to the way he used to play.
So I will say Agassi was a better ball striker which also made him a better returner. Not by much though. What Federer has over Agassi is a more well rounded game and better footwork / speed / defensive capabilities.