Notion and Obsidian dominate the "power user" conversation, but they solve different problems. Notion is a shared workspace: databases, permissions, and pages that teams live in together. Obsidian is a personal thinking tool: plain Markdown files, backlinks, and a graph of your ideas.
Ask yourself three questions. First, is collaboration central? If you need teammates editing the same docs with granular permissions, Notion wins by default. Second, do you care about data ownership? Obsidian's files are yours, offline and forever. Third, do you think in structures (tables, boards) or in connections (links, graph)? Notion excels at the former, Obsidian at the latter.
Many people end up using both: Obsidian as the private brain, Notion as the team workspace. That is a perfectly reasonable answer — as long as you know which tool owns which kind of knowledge.
For a deeper comparison, see our side-by-side comparison page, and check each app's full profile for pricing and platform details.