You're the engine, so remember that as the "first factor" regardless of what's under you. Second, weight is a bigger factor than stiffness. I'd rather climb with a 15lb aluminum bike than a 23lb carbon bike any day. Third, it's easier for me to lose 3lbs than it is to make a 23lb bike become a 20lb bike (cheaper, too). Fourth, I only hear sprinters fussing about stiffness, rarely climbers.