When DPs were first introduced the rights granted for DPs were Tiered...one year you got the right to recieve your "spouses" er "Partner's" health insurance, the next year you got the responsibility to file state taxes with your wife. . .oops Partner. (And let me just say from personal experience what a bitch! I had to file individual Federal for both of us, create a "fake" federal as if I was married and use the fake federal to fill out a real state. I lost money at the federal and state level. . .but that is a whole other topic.)
Marriage rights are instantaneous. Once you turn 18 and show that 1) you are not related and 2) the person you want to marry has opposite genitalia at the time of marriage then you are married.
Here are some other "subtle differences:
You go to a DIFFERENT location to register as a domestic partner (versus obtaining a marriage certificate).
Domestic partner registration fees are $33 (10 for the certificate and 23 for a training program on "domestic partner abuse")
http://www.sos.ca.gov/dpregistry/forms.htm Marriage licenses (at least in my county...strangely enough these fees are county based") are $49.
YOU SAVE MONEY IF YOU ARE A DP! And by the way, you can do both! I am officially a DP and Marriage participant in California! I think the supreme court needs to figure out that snafu.
But in all seriousness, what prevents lawmakers or private citizens with the required # of signatures to eliminate a right of DP (say for example the right to adopt). If you separate people into 2 groups who are in essence getting the "same right" what prevents someone from slipping in a law to say that "that group shouldn't be doing that!"