I think they are catering to different needs:
Clickup is meant as a very customizable project/process management tool. They do have a Docs feature, which is also collaborative, but nowhere as comprehensive and rich as Notion.
On the other hand, Notion started off as a Wiki software, as a tool for recording information and sharing it among a team. Again, you can repurpose it as a project management tool, but it's way less opinionated compared to something like Clickup/Asana.

I think both of these software could be used in conjunction

I'm not a deep user (yet) in ClickUp but I gotta say that it feels way more like a specialized tool for customer success - the ability to automate workflows, label everything and analyze workload sof people is very impressive. And we're only using the free version. ;) When it comes to advanced wiki features and doc building then Notion seems way better, though.

Somewhat related: I wrote an article comparing Clickup and Notion (and other tools) specifically for the task dependency use case.

ClickUp has better collaboration and is infinitely customizable, but has a higher learning curve and I found it took too long to set up to get it right. Notion is hands down better for docs / wiki and to get up and going faster vs fiddling with settings. With Notion's recently released API , I expect to see a lot more soon.