I'd say depends on your use case. It is noteworthy that the X16 has a full touchscreen that should be better for drawing than the Zephyrus Duo's ScreenPad. I cannot find what stylus support it provides though. The ScreenPad is surprisingly good for using with an MPP2 supported pen, which has to be bought separately. I happen to have that Zephyrus Duo configuration (well, I had 2+2TB SSDs, 32GB RAM and 6900HX CPU) so I can speak mainly of that. I personally like the extra screen real estate on the lap, but it would be of less importance if I had external screens. But it is a very high-quality extra screen that makes me with the main screen was 4K to match it. For personal taste, I do not mind the keyboard being at the front edge, but I know there are people who dislike it.
The 3080Ti is a top tier card so GPU upgrades will not likely be an issue. But the lack of USB4/Thunderbolt on the Zephyrus Duo might be. 3k is a great price for the performance it provides. The cooling is excellent (thanks to the lower screen tilting up), so the X16 may have trouble keeping up when speaking purely of power. It actually power throttles instead of thermal throttling. But it also means you cannot use the laptop in headless mode, i.e. when the laptop is closed, it is shut to prevent heat damage (almost no air intake when the lower screen is laid down flat). The X16 can keep running with the screen shut off. This can be useful if you prefer using external monitors when at your desk.
Another thing to note is the whole AMD-Intel issue. ASUS is better at optimizing power for AMD so you will get good battery runtimes on the Zephyrus, but you will also get hit with more driver issues. For example, because the Zephyrus has an AMD motherboard, it also has a MediaTek WiFi card, which often fails to run after waking from sleep. Other manufacturers like Lenovo have updated drivers that fix the issue, but ASUS has so far not provided any. The drivers also need manual installation every time you freshly install Windows, otherwise you will have a hard time getting online. Intel has fewer issues of that nature, but you may want to check the battery runtime tests of the X16 online to see how it does, it may be difficult to use as a mobile workstation.
As far as upgradeability goes, apparently both of them come with two PCIe4 M.2 slots for SSDs and two upgradeable RAM slots, neither wins that round.
