You should have both. One is for network wide effects, so anything on your network will have ads blocked (smart TV, projector, phones etc.), another is for youtube and friends.

Ad blocker extensions only block ads on the browser they're installed. Pi hole would block ads on all devices they're connected in, in all apps (not just web browser) as long as the ad domains they requested are in the blocklist.

Also, ublock origin can only protect you from CNAME cloaking (a commonly used to evade ad blockers) on firefox. It can't protect against CNAME cloaking in chrome or other browser. But pihole can.

You should still install an adblocker in your browser though as some ads are loaded from the same domain they're serving site contents from, such as youtube ads. Ad blockers can remove ads based on full urls, not just domains.

If you’re willing to add and maintain an ad-blocker in every single of your browsers (and potentially your family’s) and don’t care about non browser apps it’s virtually the same.

My understanding is that PiHole stops the ads from being downloaded in the first place, hence the increased network performance.