First off, most games aren't optimised for such high resolutions, meaning that the image is simply upscaled, reducing the overall quality, and second, even if it is, the fairly orthodox nature of mobile game graphics means that the difference just won't be very noticeable for the typical eyesight.
There is another side to the graphics quality story, though, as per Alexandru Voica, a technology marketing specialist at Imagination Technologies, and it is the good ol' GPU brand. The PowerVR models used in Apple's proprietary chipsets, come with a higher quality texture compression standard, resulting in better-looking imagery, regardless of the resolution. Another fly in the ultra high-res ointment is that games on the iPhone render at a higher resolution, then scale the image down to fit the screen's pixel number, which is also better for the resulting graphics quality than the upscaling that goes on with the Quad HD display of the S6.
Other factors that influence the graphics' excellence are pixel storage formats, different anti-aliasing techniques, and floating point precision. All of these components are better presented on the custom PowerVR GPU line that Apple uses, compared to the generic ARM Mali brand that Samsung is incorporating in its Exynos chipsets. In a nutshell, the examples show that the record pixel density of the Galaxy S6 doesn't really translate into better game graphics, when compared to a lower-res screen, paired with a quality pixel-cruncher.
source: Alex Voica (Techspeak blog)