It’s kind of an apples to oranges. i2p is better at protecting your privacy in my estimation, but private trackers can be better at curating high quality torrents for example.

I don’t know of any private trackers on i2p and have just used Postman mainly (the other trackers have very limited content). Postman is OK but not nearly as good as regular private trackers. If what you want are blockbuster films and the latest TV, you’ll be fine. Anything particularly nerdy you will probably find too.

Really depends on how much of a niche the things you are looking for are. Content is quite easily to search with postman. As I personally don’t really use private trackers I don’t have a lot to compare to, but most torrents I have stumbled upon had a healthy amount of seeders on i2p