If you were to expand your domain by adding on more servers to host an expanded number of users, then you should also look at modifying your FMSO roles holders and plan your DNS server structure.
If you were to go with trusts, then you would not have to worry about these issues but you would have more domains to worry about and that can bring its own headaches. To be more specific, you can have issues in one domain that has nothing to do with the other. While this is great for segregation of issues, it also means less clues as to what is going on and more potential headaches.
I would say go with the parent-child structure as this makes for easier management in my opinion. You just need to also take a look at your DNS server layout and modify your FMSO roles servers cause now your main server will have more traffic