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If your business or organization need to create security boundaries, then additional domains is what you choose. Do not forget that a domain is a security boundary with its own network resources such as users, computers, printers, GPOs,,..etc. For example, if your Finance department need resource separation from other departments, consider a new domain for the Finance department.
Again, it is all about security. Creating additional domains is a real headache to manage. I've dealt with many enterprises that use only ONE domain with several sites. Just use multi-domain if necessary and as per the business requirements.
The question is, what do you want to do exactly that made you ask this question? Give us more details about your considerations.
We make new site (that include ADC) if the second site will be managed by you and/or another admin at the other site, in this case you should have good WAN speed.
We make new child domain in case you want to seperate the management, security boundaries.