Definitely image quality. Something like Half-Life 2 ran at a really high resolution is gonna look nicer than a modern game that has aliasing and visual artifacts everywhere ruining the image. Upscaling tech will increasingly help improve image quality over time since now you can get a game that's running at 960p upscaled to 1440p that looks significantly nicer than a native 1080p image. Eventually it'll get good enough that even low-end hardware will be able to handle 4k decently even if the internal resolution is really low.