Working with Boeing on a Delta 5 (call a special version for NASA the Ares 5 if you want, but I always thought Ares would be a better name for the program to land humans on Mars) sounds like a better idea anyway. That way we inherit as much of the commercial tech as possible, especially the RS-68. A new core with five 68s (could use the ET for that, but we probably want a redesign for structural support to handle upper stages) and the Delta 4 segments as boosters would be much more reliable than the SSME + SRB. The J2-X may be usable, but I'm not sure if a second stage wouldn't be better served by using another RS-68. Plus an all LOX+LH2 launcher would be much more environmentally friendly. The SRBs are dirty fuckers. Oh, and the external boosters could be upgraded to reusable flyback boosters in the future, if such a thing is actually worthwhile.