To me Coda and Notion are two very different animals with different use cases. Coda is an extremely powerful visual database system slash application builder, whereas Notion is more of a rich knowledge portal.
Sure, Notion can notionally (see what I did there?) do relational databases but all the parts don’t integrate to a more powerful whole imo. For example, one of the example layouts is a company portal which provides you a place to enter vacation days and a separate place to manage a team calendar - but can the team calendar show the vacation days from the table? Not that I could see in the admittedly brief time I spent with it.
However, Notion looks to be stronger at attractive page layouts and rich content than Coda, so the two systems compliment each other quite well. I could see myself using Coda applications embedded in Notion - I think that would be really quite nice.
I’m not knocking Notion here. I currently use DevonTHINK for my knowledge-base and am considering switching that to Notion. I guess my point is, erm, horses for courses?