I guess I'd ask what features are the most important to you. The newer phone will be faster, have a better camera and a faster refresh rate screen, smaller front camera cutout, more ram and bigger battery, 5G... those things are to be expected.
On the other hand, an S10+ is cheaper. The screen and cameras are still excellent. It still has a headphone jack and a Gear VR support... those features might be important to some. If it's not you, personally I think the savings are not significant enough to sacrifice the latest updates in the S20 line, so I'd go with the S20+ in this scenario.