Let me see if I can clarify this a little.
For FAFSA purposes, you can't simply choose which parent submits the FASFA. The information provided should be from the parent that you live with, or in the case where that is equal, the parent that provides the majority of your financial support. If this is your dad, you will also be required to enter your stepmother's information as well, which may raise your EFC. See this Federal Student Aid publication for more on this: http://studentaid.ed.gov/sites/default/ ... parent.pdf
In terms of taxes, it's a similar deal. Only one parent may claim you as a qualifying child, and this should be whomever you live with. See this similar post for more on claiming a qualifying child on taxes
For Parent PLUS Loans
, the borrower can be either parent, it does not have to be the parent that provided their information on the FAFSA. Both parents can take out these loans, providing the combined amount does not exceed your cost of attendance.
As for who should take these loans out, this is something your family will need to solve.