If you have a lot of really steep or long hills on the route, and one has considerably lower gearing that the other, use it.
If it's going to be cold at the start and hot at mid-day, and you need to carry two bushels of clothing to make that work, use whichever can carry that extra cargo.
If the racier bike makes your hands go number or hurts your butt or something, or regularly pops spokes or something, use the other one.
Otherwise, I would vote for the faster lighter bike.
I've ridden steel, titanium, carbon fiber on 200k's, they're different, but the main differences are in racks and lights and not so much in the frame itself.