Lets take a look on the specs of both the smartphones:
The Samsung Galaxy S20+ is slimmer which is good and easier to carry in the pocket whereas the oneplus 8 pro is bigger in size which makes it good for reading and big screen.
S20+ is lighter in weight than 8 pro. But oneplus 8 pro has much better camera resolution. It has Quad Camera.
So if you are interested in photography and want a good phone that clicks good pics then oneplus 8 pro would be good.
Also one plus 8 pro is having good battery life.
Both have almost same memory configurations.
Talking about CPU 2.73 GHz in Samsung galaxy S20+ and 2.84 GHz in One Plus 8 Pro.
One plus will support fast charging too.
So the differences are very few, and if we see the cost the samsung is a bit costlier than one plus 8 pro.
Many similarities are there, but camera of one plus is one of the feature that is exception in 8 pro.
If budget is small, then I would recommend you to go for one plus 8 pro but you are a samsung guy then you can go for galaxy S20+.
Hope this helps you.