akkelly, i'm thinking neither one. in a situation where you have both a father and a stepfather, i think people would understand. if you really want someone to walk you down the aisle, how about your mother? sounds like she's been one of the few "constants" in your life, KWIM? or do you have any other people who are close to you (an uncle, cousin, brother, best male friend?) whom you didn't include in your wedding party? it could be a nice way to honor them.
there are many threads on this dilemma too, if you want advice from people in similar situations. bottom line is you have to do what will make you feel most comfortable, and that means avoiding drama and following your heart.