If I had to choose only one, I'd probably go with Obsidian, but I haven't found a free mobile/desktop syncing option that I've been happy with. Both Syncthing and iCloud have given me file versioning conflicts for some reason, so I'm always hesitant to use it in case I were to lose something. Notion just works without me having to think about it, and I do actually like the way it handles some things, but I like how flexible Obsidian can be with plug-ins. Until either app does something to push me away, I'm fine just floating between the two for now.