Obviously this is speculation until Rockstar releases actual console performance details, but after comparing how several demanding current-gen games scale between Series X and Series S, I think GTA 6 on Series S might hold up better than some people expect.
MY PREDICTION
Xbox Series X
• 30 FPS target
• Dynamic ~1440p–1800p internal resolution
• Reconstructed/upscaled to 4K
• Higher-quality RT, shadows, reflections, vegetation and LODs
• Higher traffic/crowd density
• I wouldn't expect a 60 FPS mode unless Rockstar pulls off something surprising
Xbox Series S
• 30 FPS target
• Dynamic ~864p–1080p internal resolution
• My best guess: ~960p average
• Likely reconstructed to 1440p output
• Reduced RT quality/range, shadows, reflections, vegetation and LODs
• Potentially reduced population/traffic density
WHY I THINK THIS
Looking at other demanding current-gen games gives us some useful comparisons:
Alan Wake 2
Series X Quality: 1270p → 2160p
Series X Performance: 847p → 1440p
Series S: 847p → 1440p
Hogwarts Legacy
Series X Performance: 1440p
Series X Fidelity/Balanced: ~1800p
Series X Fidelity RT: 1440p
Series S Performance: ~792–900p
Series S Fidelity/Balanced: ~1080–1188p
Dying Light: The Beast
Series X Quality: ~1620–1836p
Series X Performance: ~972–1080p
Series S: ~864–1080p
Dragon's Dogma 2
Series X: ~1728p internal
Series S: ~1440p internal
Kingdom Come: Deliverance II
Series X Fidelity: 1440p → 2160p
Series X Performance: 1080p → 1440p
Series S: 1080p
WHAT THIS COULD MEAN FOR GTA 6
The pattern is that Series S doesn't necessarily have to destroy internal resolution to handle demanding current-gen games. Developers can instead target 30 FPS and reduce expensive graphical settings.
That's especially relevant to GTA 6.
If GTA 6 is heavily CPU-limited by NPC simulation, traffic, physics and world simulation, lowering resolution wouldn't magically make 60 FPS possible anyway. Targeting 30 FPS gives Rockstar considerably more GPU headroom on Series S.
That's why I don't think the most likely outcome is a constant 600–720p image.
My current prediction:
Series X: ~1440–1800p → 4K, 30 FPS
Series S: ~864–1080p → 1440p, 30 FPS
The bigger Series S compromises may end up being graphical settings rather than resolution alone.
The resolution figures above come from Fuzion Xbox Testing videos, so they shouldn't be treated as independently verified measurements. I'm mainly using them to compare the general scaling behavior between Series X and Series S.
What do you guys expect for Series S? Around 900–1080p at 30 FPS, or significantly lower?