Been using a PiHole for over a year. Works great. Had never heard of NextDNS until now but I guess they’re newly on the market?
Hope this isn’t a deep state organization setup in time to support contact tracing and control. 
Just tried it on my iPhone. Wow, quite powerful. What I especially like over pihole is that it’s cloud based. I’m using my Pihole at home but on mobile I was using PrivacyPro from Disconnect at a yearly cost of $32 CDn (for more advanced features, ad/tracker block is free)
Using NextDNS frees me from that subscription and having to maintain a Pihole if at some point we can setup on our Peplink Surf router.
The other major NextDNS win over PiHole for me is parental controls. It’s got a ton of features. Enforce Google SafeSesrch, YouTube restricted mode, block child abuse sites, whitelist domains etc.
Analytics are great.
Downside is for iOS, requires app install and VPN.
This is ok for young kids, as you typically wouldn’t allow them to uninstall the app but wouldn’t fly for older kids or teens.