No. 1: Josh Giddey
It will take time for Gordon Hayward to work his way into what the Thunder are doing this season. He will need to pick up the offense, practice with his new teammates and ramp his body physically into game shape after missing weeks with a nagging injury. When he does debut after the All-Star Break he will have a smaller role coming off of the bench.
Eventually, however, the upside of adding a player like Hayward is that he can play with the other starters, be that as a starter, a closer or both. Hayward is a gifted scorer who will take some amount of shots, but he is also a point forward whose ball-handling and playmaking for others will be a boost to any lineup he is in.
When you factor in his shooting, Hayward is in essence a more proven and more versatile version of what Josh Giddey is bringing to the Thunder's top lineups. Giddey is a better rebounder and would be a better choice to run his own team, but as a player looking to thrive with the other starters, Hayward is clearly a better on-paper fit.
Josh Giddey won't be completely punted from the rotation, but his lack of a jumpshot allows opponents to leave him wide open on the perimeter. He can develop counters to that, including sprinting out to set screens outside of the arc (a la Draymond Green when he is left uncovered) but right now Giddey is a hindrance to those lineups as often as he is a help.
Gordon Hayward, if healthy, is a much better fit. He has a lot of playoff experience, can fit in nearly any lineup, and has the skillset to be on-ball or off-ball as needed. As Hayward gets acclimated and as the season moves toward the playoffs, it will not be a surprise to see the former All-Star take Giddey's role or even his spot in the starting lineup.