I don't think I'd buy an overly stiff frame aluminum frame for long distance. There are some really good aluminum frames out there but they are pretty expensive. You do want good power transfer with minimum flex but you also need to get to the finish line with your teeth still in your head.
Bang for buck I think you're best off with a decent carbon frame.
One note about aerobars and long distance. If you go to the start of a long distance event you are going to see the vast majority of racers have aerobars on their bikes. If you look a little closer at the way they have them set up they are not slammed down in a full aero position. They tend to be set up a bit higher to allow better breathing, comfort and still retain a more aero position. See the diference between these two racers: The one in blue is in a typical TT position. You can see how flat his back is and how low his bars are. The rider in red is a RAAM racer. His bars are higher and he isn't quite as leaned over. It's actually quite a comfortable position for long distances.
