2 alum bikes. Design and geo makes the biggest difference. One alum frame replaced with a mostly carbon frame, same geo and design, no difference in feel. The frame tha snapped, an aluminum frame.

As far as fatigue, if I do a century on the aluminum bike without proper training, I feelm fatigue. If I do a century after some good preparation, I feel zero fatigue. Conclusion, get into shape, the alumimun will ride like carbon!

When do you "need" a carbon bike? As soon a you can afford $1500 climbing wheels to help you over the fwy overpasses!