I have both, XPlane11 and FS2020, and I find myself flying FS2020 almost every time.
XPlane11 is more responsive to control inputs, making it harder to fly, but not necessarily more realistic. I find that I am over-controlling far more than in reality in a C172. FS2020 is more docile, but that makes it on the other side of the spectrum as far as controllability goes. Of course you can play around with sensituivity settings and such, but I have not been able to get it to the sweet spot with XPlane11, while FS2020 comes close.
I fly sims for fun, but also for training purposes, so that's why I am looking for realism first.
As far as graphics go, FS2020 is a clear winner. XPlane can get every bit as beautiful, but that requires addons. They may be free in a lot of cases, but they sure put a burden on your system resources. FS2020 is better in that respect, "out-of-the-box". Also, with BING maps, VFR flights around the entire world are better than with OOTB XPlane11.
Downside is that you will need a "trick" to get Flight Logger to work with FS2020.
With X-Plane, there is XP-UIPC, and FL was developed when XP9 was already on the market, so that simply works.
For FS2020 you need FSUIPC7, AND you need to have run FL succesfully against a "known" sim first at least once, on that same PC. Here's why: On startup, FL looks in the list of processes which sim is running, and looks for any update in scenery files since it last started. If scenery files are updated, FL will re-scan those files to determine if ICAO codes were changed or added, for instance. So, if you have a fresh PC, with only MSFS2020 on it, your fresh FL installation will not recognise any sim running. Normally FL will then take the last known database (the one from FS9, FSX, XP9, ZP10, XP11, P3D) and use that one, assuming that you started FL before the sim. On a fresh (or a re-)install, that "last known sim" database does not exist yet. That's why you need to have run FL sucessfully against one of th known sims first, before FL can use that other database. Note that if you have run FL against FS9 earlier, FL will use FS9 ICAO codes. Which should work fine because the website converts between the sim formats, but it IS annother risk there.
But other than the potential FL issue (it has been working fine for me with FS2020 for half a year now), I would recommend FS2020. Better default scenery, better visuals, very regular updates, good "tweaakbility", of course loads of addons - also in freeware there are some awesome things (I think FS2020 community addons are at the same level of XPlane11 already, and increasing in number by the minute).