[1] "We are making Microsoft Flight Simulator. Emphasis on the word SIMULATOR." https://www.flightsimulator.com/june-20th-2019/
XP11 focuses on the flight models and physics of flying a plane, whereas MSFS2020 focuses on the visuals.
Neither are perfect, but for those studying or familiarizing, XP11 is a far better choice since it goes out of it's way to mimic real life. MSFS2020's basic G1000 (crammed into nearly every cockpit in the game) still has buttons that don't function, just for starters.
Different focuses, one's a "game" and the other is a "simulator".
Basic things like the inability to manually set barometric pressure - there's no option in the G1000 implementation, you have to press 'B' on your keyboard and it just sets itself. Little things like that add up and firmly take MSFS2020 out of the "simulator" category... for now.
Some others have mentioned that MSFS2020 might be more akin to a platform, upon which content will be added over years and years. Some enterprising individuals have already figured out how to add helicopters, for example, while officially there's no support for helis by MS/Asobo. So, I remain hopeful.
So, I think MSFS2020 will be a great simulator for those hardcore simmers who are willing to pay 50-150USD for a single aircraft modeled to a very high detail, just like they did in FSX days.
Whereas, the flight simulator that Airbus trains its pilots on is evaluated by a different goal... that simulator's goal is to make sure that you don't kill 400 people on an A380. You're not supposed to enjoy it, using it is paid work and not recreation.
Now, one could argue the differences between X-Plane and MSFS in terms of realism. From a professional point of view (airline pilot or aerospace engineer,) both are very much in the same general field. Truth is that neither of them meet performance requirements to be used in the kind of certified full motion flight simulators airline pilots use on the job, although both products arguably have components that are useable in those. However, both products are also highly extensible. Just because the out of the box models may not be super detailed doesn't mean an add-on product can't come with its own flight and system models--for instance, PDMG and FlightSimLabs produce airliners for the old FSX and Prepar3D that will outperform most everything on X-Plane from a realism perspective. Expect that their products will be on MSFS2020 in time.
Also, it's important to distinguish between flight model realism and systems realism. Both X-Plane and MSFS add-ons can and do come with their own flight and system models which replace out of the box componentry, but arguably it's the flight models that are more dependent on the capabilities of the base sim. Everyone complaining about the lack of realism in the MSFS stock G1000 implementation should just wait 6-9 months, and surely something better will be out.
From my POV, the new MSFS does 2 extremely impressive, arguably revolutionary things, and the aerospace/aviation industry has taken note: a) The photogrammatry rendering of the whole earth in high fidelity 3D detail, and b,) the representation of the real ATC airspace through leveraging ADS-B (i.e. FlightAware and Flight-Radar 24.) Simulating a single airplane to a high degree of fidelity on desktop hardware is easy. Simulating (I mean truly simulating) an airspace of airplanes is not... ATC in most sims--to date--is scripted, not fully simulated. Piping in the real world (and combining it with live multiplayer) is a very interesting alternative.