I use both Notion and Obsidian. The main difference is that Notion is an entire database processing platform, especially once you get into mirroring databases, and getting savvy with filters and check boxes. Not to mention the fact that everything in Notion is its own page. This makes it wildly customizable, and again, programmable.

Obsidian is a lot more static. It's basic strengthis having offline capabilities, simplicity, and markdown. I use it exclusively for my Zettelkasten, which I also have on Notion (I started my ZK and migrated it over to Obsidian). Obsidian is great for ZK in part due to its simultaneous multiple panel openings. So, you can see multiple notes at once. Notion can do this too, and I've use its Gallery fxn for exactly that, but there's something less claggy about Obsidian in this regard.

Notion is great for GTD and project management (again, when used dynamically with filters etc). Obsidian is great for note storage and retrieval, as well as processing ideas and arguments for use later in writing articles, scripts, etc.