The situation is looking pretty grim for the Xbox One: http://arstechnica.com/gaming/2014/02/next-metal-gear-solid-runs-1080p-on-ps4-720p-on-xbox-one/ The evidence suggests it is not suited for 1080p game graphics. The 32MB of super-fast eSRAM is also super-small.
Another reason why SteamOS will be superior. Its just a PC with a sweet video card, so it can handle 1920x1080 easily.