If you entered and were inspected upon entry, in most cases you would be much better off marrying then adjusting your status, as if you were to leave first you would be subject to a potentially long period of exclusion before you could lawfully reenter the United States.

You should never take legal advice from the USCIS - they can at most give you legal information, and they have a very different agenda than the immigrant. You should get legal advice from an immigration lawyer.