So, anecdotally, I used pihole first more than 5 years ago and switched to AdGuard as pihole did not have the ability to do conditional forwarding of requests for various zones or the ability to add static records via the UI. Conditional forwarding means that I can send the requests for let’s say example.com to an internal server hosting that zone responding with private records for internal services as well as other similar scenarios.
I also like that I can identify clients or networks in adguard by various factors and apply different rules (blocking and forwarding) and collect statistics on those clients or groups of clients, I don’t think pihole has either feature yet.
I also like that adguard is a static binary which is likely what people mean when they say it’s easier to install and maintain.
As to why I keep it and don’t switch back, I like the interface AdGuard has and it doesn’t break so I often forget about it anymore. I’ll update if I remember anything else but those are the larger things for me. If pihole is working then stick with it but curiosity is a definite reason to try adguard, I bet you could just stop pihole on your machine and run adguard to check it out without too much work (yay static binary) but I haven’t tested that myself.