Phoenix and Apollo both shine in their own ways. Apollo is a bit more calculated than Phoenix is and seems to be a bit more serious as well, especially when we see his internal thoughts. Phoenix, on the other hand, is a total goofball. His AA3 flashback self is very much alive inside of him. What I LOVE about the new trilogy of games is how when we see Phoenix from an outside perspective, he is calm, composed, cool, and confident. Yet every time we are given a glimpse inside his head he's still doing his "fake it til you make it" deal. It cements him as the ultimate bluffer.
On the other hand, Apollo comes off as the goofier one to others, being afraid of heights, yelling to calm himself down, etc. They're different characters and playing as both of them makes them both shine more brightly.