Yeah, definitely more similar functionality in AdGuard vs pihole, but similar significant difference. Pihole protects only when you are home and protects all devices, even ones thst can’t install ad-blocking like streaming devices and such. AdGuard protects the device even outside of your home. Doesn’t hurt to use both.

But I prefer rethink over adguard since it’s open-source and so it’s not as easy for them to go rogue for profit and start collecting personal information. Also, I’ve never used adguard but the website says the free version only protects browsers and not apps. Rethink protects the whole device even in DNS-only mode. And adguard seems not to offer as much detailed control of what’s blocked, though I may have missed it. Rethink you can download several kinds of blocklists and either apply the whole list or select site-by-site. But that’s just my preferences.

Either way pihole and adguard serve different purposes, pihole is network level and affects more than just personal devices and adguard works even when outside of your home, but only affects personal devices.