DeNotes vs Obsidian
Obsidian and DeNotes agree on the most important thing: your notes should be plain Markdown files you own. They disagree about how much of the tool you should have to build yourself.
What Obsidian does well
Obsidian is the most capable local-first Markdown app there is. Your vault is a folder of files on your machine, the plugin ecosystem is enormous, and the graph view, backlinks and community themes let people build something exactly suited to how they think. For anyone who enjoys tuning their tools, it is hard to beat.
Where DeNotes is different
Obsidian starts as a blank instrument and rewards the time you spend configuring it. DeNotes starts finished. There are no plugins to evaluate, no themes to install and no sync method to decide on, because notes go to your Google Drive by default and are there on every device you sign into. DeNotes also runs in the browser and installs as an app from there, so there is nothing to download first. The trade is real: DeNotes will never be as extensible, and it has no graph view or backlinks.
Should you use DeNotes or Obsidian?
Choose Obsidian
Choose Obsidian if you want to shape your own system, care about backlinks and the graph view, or want your notes on local disk with no cloud account involved.
Choose DeNotes
Choose DeNotes if you want the same plain Markdown files without building the tool around them, and you would rather sync just worked through storage you already have.