@Boethetic
Some of the device hardware/feature differences can be found at https://en.wikipedia.org/wiki/Roku
Similar spec comparisons can be found at Roku developer page https://developer.roku.com/docs/specs/hardware.md
----
The 3820 does have a newer processor and more channel storage. An issue with how fast any of these devices are when released can change significantly when a new OS update is released by Roku. But generally, I would say you will probably a better experience using the 3820 over the 3810. It really depends on what kind of content you are streaming. (ie. channels/apps).
Here is a little image comparison compiled from the above wiki link of the 3810/3820.
click on image to enlarge.

Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660, 4802, 4850, Premiere+ 3921, Express 4k+ 3941, Streambar 9102