I'm a Florida local, so I am very used to the fiery heat of summer (
ESPECIALLY in the hottest park: AK). If you aren't really used to the heat, especially that AK summer heat, I would go with the 8:45.
Those two times are pretty close to each other. The heat difference between these two times will not be totally drastic. However, the 8:45 time would be cooler especially in July. Again, it will not be dramatic difference in the heat, but it will be cooler than the 9:45 time.
Drink lots of water and enjoy!