Should I add DNS records using Linode DNS Manager or ISPConfig?

This depends on your ultimate goals and the needs of your applications. While software like ISPConfig/cPanel may make it easy to manage your own name servers and automatically add DNS records, the software is still running on your server and the maintenance/configuration ultimately is your responsibility. On the other hand, services like Linode's DNS Manager will take care of managing your name servers but you will need to configure your domains and DNS records yourself.

You might also want to consider the built-in redundancy and performance benefits of using services like our DNS Manager. Here's a community post from a few months ago that provides more details on how to choose a DNS Manager:

https://www.linode.com/community/questions/17607/which-one-is-better-adding-a-record-or-add-dns#answer-67799