What is more beneficial career in long term, Product Owner or Project Manager?

It what you want actually matter. Do you need more money? Or more stability? Or less work pressure

Product Owner

Start with 1 product, become product owner of 1 more and then one more.

Overtime takeover all products as product owner and eventually the company 😁😁

On serious note, depends on your interest , if you are more interested in growing in same organisation or same field then PO.

If you are interested in switching fields then project manager.

I have rarely seen a project manager rising to a very senior position.

Product owners do rise but then they are not career product owners but use PO role as middle stage in their long career.

Disclaimer: Never worked in new age product company so views are coloured