They are very similar. The main differences are:

• LogSeq uses bullet points. Obsidian is just pure markdown
• LogSeq is open source. Obsidian is closed source
• LogSeq has a predefined structure to it (folders). Obsidian allows you to have whatever folders you want

Personally, I use LogSeq for my day to day work. Primarily because I prefer the bullet point approach when taking notes. But some people would prefer writing long continuous text with Obsidian.

So to each their own. If you're interested, try both (they're both using markdown, so you can transfer between the two). I went back and forth a few times before settling with LogSeq