Overview
Ultimate SD Upscale does not preserve every pixel. A low denoise value still allows the model to alter details. For example, the subject's gaze can shift slightly between the direct upscale and the tiled result.
When exact preservation of the original image is the priority, a conventional model upscaler is the safer choice. When sharper and more generated detail is more important than exact fidelity, tiled image-to-image upscaling is the stronger option.
Tile seams can appear. Mask blur and padding address that failure mode; start with their default values and adjust them when seams remain visible.
Two-stage upscaling creates more graph complexity and computation than a single direct pass. The additional work is justified only when the detail improvement matters.