In your opinion, which is better to buy Asrock N100DC or Asus Prime N100i-d d4?
It depends on what you are looking for.
In my opinion the main difference is that the Asrock motherboard forces you to use an external power supply. The motherboard has a built-in power connector for two hard drives, so you'll be limited to only two mechanical hard drives unless you find another way to power a third hard drive.
The Asus motherboard allows you to use a standard power supply, so you will have no problems powering the number of drives that you are able to connect to the motherboard.
On the other hand, I seem to remember that the Asrock motherboard forces the CPU to work a little above its specifications. This will give you slightly higher performance but could also reduce its useful life.
I see that there is also an Asus Prime n100i-d d4-csm, but I don't know the difference between those Asus mbo
The only difference is that the second includes management software.