If you use a subdomain it will not be much different from running a separate site.
Instead, run the acquired site for long enough to understand the traffic flow and its income potential. It may be the more valuable site.
If you do merge, do it carefully by redirecting related pages and keeping all of the content on a single domain (not a subdomain).
* "It's not the size of the dog in the fight that matters, it's the size of the fight in the dog." Mark Twain
* "Free advice isn't worth much. Cheap advice is worth even less." EGOL