Here's my take on long distance bikes having ridden 1200k's on steel, Al., Ti and Carbon bikes and RAAM on Carbon and Al. bikes.
First off, frame material should be way down on your list of priorities when looking for a randonneuring bike. You have to look at your bike as a package and everything is interrelated. The first thing you have to do is to decide exactly what you want your bike to do. That's why the advice to use your current bike for a while (at least till you've done a 600k) until you've learned exactly what you want your bike to do and not do. Comfort should be your number one priority. You can't be faster if your are miserable.
Here are some things I've concluded about frames for long distances.
1. Fat tires are a crutch for poorly designed frames. I don't mean expensive frames, I mean poorly designed.
2. Get fitted. Let your body choose your bike.
3. Carbon Fiber frames do dampen some road vibrations better than other frame materials. From my experience, its pretty much irrelevant for distances less than 200-300 miles but as the miles pile up it gets more pronounced.
4. Touring geometry is for touring. If you want to carry handlebar bags, panniers etc. then a touring bike is for you. If not, it's not necessary. You can be just as comfy on a race geometry bike if you're fitted properly. btw, a lot of people get away with lite handlebar bags on race geometry bikes with no ill handling effects.