Unlike the PS4 Pro, Microsoft decided to include native support for a resolution used by gaming-focused monitors.
As far as Sony is concerned, we all game using either a 1080p or 4K display. But there is another fairly common resolution used by monitors, and in particular gaming-focused monitors. It's 2560-by-1440, or 1440p. Microsoft decided to include native support for 1440p on the Xbox One X, and that could prove significant.
If you look down a list of gaming monitors right now, and in particular those featuring Nvidia G-Sync or AMD FreeSync, you'll see a number of them use a 1440p resolution. So if you own a 1440p monitors, or would prefer to run an Xbox One X using a monitor, Microsoft has your back. However, there's no confirmation either G-Sync of FreeSync will be supported.
As The Verge reports, news of support for 1440p on Microsoft's latest console came via Kevin Gammill, Xbox Program Manager. He confirmed native support through his Twitter account.
If you're wondering what the PS4 Pro does when hooked up to a 1440p display, it upscales. You'll get a 1080p resolution upscaled to 1440p, which will look decidedly worse than native 1440p output.
In the grand scheme of things it's a niche feature, but one that will be appreciated by those who already own a 1440p display. It also counts as a feature Microsoft can use in marketing against Sony, and with the market being so crowded this holiday season, Microsoft needs every advantage it can get.