If your girlfriend is already a citizen, it might be a few months faster if she filed for you because you would be able to file I-130 and I-485 together right away after being married, whereas with your father you have to wait a few months for him to become a citizen before you can file the I-485.
However, marriage-based cases involve much more scrutiny and documentation, so the marriage-based route could actually take longer if they decide to investigate your case more deeply and demand extra documents or schedule a second interview. And once approved, initially you would only get a 2-year conditional card, and then about 2 years later you have to apply to extend to a 10-year unconditional card. If you get divorced before the 10-year card is approved, they may deny your 10 year card and deport you.
Whereas if you file I-485 based on your father's petition, upon approval of the I-485 you would directly get a 10-year card. And there might not even be an interview. If there is an interview it will be a simple one, nothing like the invasive marriage-based interviews where they ask all sorts of questions about your private life. But if you were born out of wedlock, it can be a bit difficult to provide documentation of the paternal relationship; you may have to provide a DNA test and/or evidence that your father was supporting you growing up (either by child support payments or you living with him), in addition to the birth certificate.