There's a significant difference in one scene, when Sage is in the mc's room and she's letting off steam, punching his gloves.
As friends, the scene plays out fine.
As fuckbuddies, she makes advances and the mc suddenly goes all cold on her, worrying about being the "rebound guy" and makes the whole scene shit. A lot of Sage fans absolutely hated how that scene played out. I mean, the mc has been fucking her all this while, and suddenly he's worried about being a rebound guy and turns away her advances in that scene.
Other than that, it's no big deal, and by episode 9 I don't think there's any difference as to how the mc ended up with Sage.