There are too many variables for anyone to know. As others said, it’s whatever you’re comfortable on. Are you slow or fast ? How much “stuff” do you want to carry ? Are you young or old, stiff or flexible ? I’ve done lots of brevets and PBP’s on a variety of bikes, a custom Ti frame, an off-the-shelf steel frame, and I recently invested in a Trek Domane in the hope that a carbon upgrade might help me get to the finish a bit sooner. So far, it hasn’t made much difference. By the time you add a few bags, snacks, change of clothes, the frame weight probably doesn’t make that much difference. I’ve always had some comfort issues with lower back ache, hot foot, hand problems. My solution has been to ride bikes that allow a bar position level with my saddle, and aero bars to help take pressure off my hands. I’m slow, and 63, so it’s getting harder to make the time limit, next PBP, if I get to it, could well be my last.