This is really a choice between design priority and all-round practicality. The iPhone Air is the type of device to choose because you specifically value a thinner/lighter feel and are willing to accept compromises that come with that design. The Galaxy S26+ is the safer all-round choice if battery headroom, camera flexibility and conventional flagship practicality matter more.
I would handle both phones in person before deciding. A noticeably lighter phone can improve every interaction, but that advantage only matters if the battery and camera setup still meet your needs.
If you travel, shoot a lot of photos/video or spend long days away from a charger, lean toward practicality. If you mostly use messaging, social media, calls and light photography and really value thinness, the design-focused option becomes more attractive.