If I were you, I'd at least make sure you have the oppertunity to leave with a masters if you want to. If find you don't like the institution so much, you can move location or career direction if you wish to do so.
I would say, do the masters first, then decide on further study. Put it this way, would you sign a 5 year contract with a company at this point in your life?
It has been suggested (on this site) that there is some sort of odd stigma associated with having a master degree in the US, as it implies you are a PhD dropout. But I don't know much about this.