It might be a good idea to look into the practicalities of your various options before coming to a decision. You say that your girlfriend wants you to move with her to the USA but the vast majority of people who want to live in the States simply CAN'T.
In order to move there you'd need to show that you'd got a valid reason, such as going to perform a job that couldn't be done by an American citizen (e.g. if you were a famous actor or international sports star) or having close family ties there. Even then, it can be an incredibly lengthy and complex task to get a residence visa. My friends, for example, qualified under the 'close family ties' rule (because their son, married to a US citizen, was already living there) but it still took OVER TWO YEARS of form-filling and interviews before they were finally allowed to make the move.
So there would be no point in turning down the job, on the basis that you'd then move to the States with your girlfriend, if such a move simply wouldn't be possible for you anyway.