I hear that a steel frame is more comfortable to ride for a long tour but that it is heavier and easier to rust than aluminum. Which material is better for a touring frame?
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Steel is still very common for relatively expensive "touring" bikes (bikes intended for long distances carrying panniers). The slight additional weight of a steel frame over aluminum (well less than 10 pounds in most cases) is inconsequential when you have 40-100 pounds of gear on the bike, the bike is more durable, and the flexibility of a steel bike is preferred by many on a long ride.
Aluminum is more rigid because the tubes must be made fatter and thicker to achieve the same yield strength as steel. In some contexts this additional stiffness may be appreciated, but generally not when riding long distances.
Stainless is hardly ever (never?) used for bikes because it's too hard to work with and the metallurgy doesn't allow the characteristics of the material to be controlled nearly as well as steel or aluminum. And its heavy.
Carbon fiber is carbon fiber. Great for lightweight frames (saves another 2-5 pounds, maybe), great for bragging rights, but fragile and often excessively rigid.
One important consideration on a cross-country ride is that a steel frame is very unlikely to suffer fatigue failure, while the other materials are considerably more likely to do this. (And, in a pinch, a steel frame can be repaired by a welder at a local auto or tractor shop, while a broken aluminum or carbon frame is likely toast.) (Of course, any good quality frame that isn't overloaded should hold up to a 3000 mile journey, but sometimes things happen.)
The tendency of a steel bike to rust is of little consequence. A steel bike would need to be exposed to the weather constantly for 20 years before rust would cause a failure, and at that point all of the components would have already disintegrated. And I don't know about carbon, but aluminum and stainless steel tend to suffer crystallization of the metal at the welds as they corrode, and this can lead to sudden joint failure.
I have toured a bit, both on steel and aluminum. I have also read a lot about the classic pros and cons perfectly explained by DanielRHicks.
So, based on study and personal experience, I would tell the overal quality and specificity of the bike is much more important than the material itself, for these reasons:
Elaborating over the problem of material failure, I can surely state that travelling, or "trekking" by bike puts a lot of stress on the frame, but mostly the problems you'll have are related to the points where luggage-carrying structures are attached, like the rear rack brazeons over the rear axle. I broke both of the eyelets on my touring bike, it is a steel one, but the frame is intended for mountainbike. The eyelets were brazed on with a tiny cross-section (I think more intended to mudguards or a light loaded rack).
Also, a touring frame could not be too flexy, because the bike could become difficult to handle at low speed (flexing on the fork), and dangerously unstable at high speed (phenomenon known as "shimmy").
My advice would be: either aluminum or steel, choose a frame that was intended for touring, with good design solutions developed with touring in mind, and whose manufactured is acknowledged for producing good touring bikes. Or at least bikes that are expected to be used as tourers or commuters (almost the same requirements). The Surly Long Haul Trucker is a good benchmark.
Aluminum tends to be stiffer, won't rust (forms an Aluminum Oxide coating), is much lighter (about 1/3 the weight, but has to be thicker for the same strength, so it usually works out to be 1/2 the weight of steel). It's extremely rigid, so it won't bend... until it breaks.
Steel is less rigid, heavier, and will rust. I would expect the 'suspension' effect of the frame to be rather minor, especially when compared with a true full-suspension bicycle, and the composition of hardtail frames to be not too relevant to your comfort. It may tend to be somewhat more durable, in certain cases - it will bend before it breaks. I find this undesirable for a frame, I'd rather it stay true until it cracks than wonder if it's bent.
Stainless Steel has the properties of steel with similar weight, but without the rusting issues.
Carbon Fiber is expensive, but very light and nearly as strong as steel and rigid as aluminum at about 2/3 the same strength piece of Aluminum.
For what it's worth, I'm going aluminum on my next mountain bike frame. I ride a steel GT Bravado now, and I'll be looking forward to the weight decrease.
The answer depends on a number factors.
In regards to your question of 'easier to rust', I have a Trek Elance 330 from the 80s which has a steel frame and not an inch of rust to be found anywhere. It has taken me many miles in full comfort, and I enjoy riding it.