Aluminum is definitely the bargain, and the builders have worked out the weld quality problems of the 1980s. I cannot vouch for how comfortable an aluminum frame would be after four hours of riding. The problem with Al is that it "rings" when you hit a bump. The shock turns into a vibration that is not damped and it "echos" around the frame. Steel, by nature, dampens that shock excellently. I understand that frame makers have been working on that problem and that the frames are much better for shock absorption these days.
Carbon fiber is perfect for dampening vibration. I really don't know about titanium, but I don't think you will be buying carbon or Ti because of price. I have a steel bike.
A longer frame is more comfortable. You will find those in "touring" bikes. The elegant long-curving rake in the fork is actually a spring to absorb shock. Touring bikes also have longer chainstays. You give up rear triangle stiffness, though, so the bike will not be as efficient and will not take hills as quickly. Your pedaling energy will actually be wasted in the springiness of the frame (!).
The last comment about comfort is that I would get "biggish" tires like 28mm wide. 23mm and such are for people obsessed with saving weight. I don't race, and I don't care. Ask the bike shop if they can switch to wider tires when the bike is delivered.
Sheldon Brown, the prophet of this forum, recommends getting a saddle with springs unless you are weight-obsessed. Padded gloves and padded bike shorts make for a comfortable day of riding too. The gel shorts get kind of sticky, though. They don't breath.