Difficult question. Dealing with proven talent, if the injuries to the D don't bite the Texans this year, they have decidedly improved upon a top half (2002) D (minus Stevens/Posey, plus Robinson/Peek/McCree/Babins/Smith--is there really a debate here?). On the O side they have improved tremendously but at this point much of it has to do with potential/durability. The safe bet goes to the D, but the high end optimism goes to the O.
As an FYI, last year's superbowl contestants had:
the 16th (Carolina) and 17th (New England) offenses, and
the 7th (New England) and 8th (Carolina) defenses.
The Art of War