Is an Intel N100 or N150 a better value than a Raspberry Pi?

I think both are great. It depends on what you need and the requirements you want to hit. I use an RPi for as a Pi-Hole for example. It works great. Low power and just that one task. Performs nicely. And cheap. However for my firewall (PfSense) I use a mini PC because I want the throughput especially when I VPN into it. Also works great for that task. So I think of it in terms of 'task' and it's footprint (ie storage/mem) and throughput.

You can't run a DNS server on the same mini PC? Seems like that would be ideal.

I run PiHole on a Pi Zero, which isn't really comparable to any mini PC in cost or performance. It uses such little resources that I'm surprised that most new routers don't offer the DNS filtering features out of the box these days.

Pi-Hole specifically has a limited number of officially supported OSes - https://docs.pi-hole.net/main/prerequisites/. PfSense/Opnsense run on top of FreeBSD which is not supported by Pi-Hole.

I assume this is true of pfSense, but Opnsense has a number of available DNS server options built into the distribution.