I’ve used both, each for a long stretch of time; they are fundamentally extremely similar and you’ll be fine with either. I switched to AdGuard Home entirely because I could run it directly from my OPNSense router instead of a second machine. There isn’t really anything else major I’ve noticed different between them, but my usage is fairly basic. AdGuard’s interface felt a bit more mature and clean, but that’s it.
If you’re happy with your PiHole, there’s no reason I’m aware of to switch.