Wait If You Want Lengthy Software Support
While it might be tempting to go with a cheaper device like the Galaxy Note 9, Galaxy Note 8, or even the Galaxy S10, software support for those devices will end much sooner.
Samsung typically keeps devices upgraded with major updates for two years before support ends. For instance, the Galaxy Note 8 won't get Android 10 and the Galaxy Note 9 might get stuck on Android 10.
The Galaxy Note 20 will most likely run Android 10 out of the box and it will pickup Android 11 and Android 12 as well.
The Galaxy Note 20's status as a flagship also ensures that you'll get the best version of Android 10 and Samsung's One UI. The Galaxy Note 20 will run One UI 2.5 out of the box.
Older devices sometimes miss out on software features due to hardware limitations and that's a risk you take buying an older model over a newer one.
Older Galaxy Note models like the Galaxy Note 10 will almost certainly get some of the Galaxy Note 20's software features, but it could take months for them to roll out. And even then, they might not get all of them.
If you plan on keeping your next phone for awhile, go with the Galaxy S20 or think about waiting for the Galaxy Note 20.