6061 T-6 has magnesium and silicon as its alloying agents, and it's ability to withstand shocks and such can be positively or negatively enhanced by loading, gradient, and surface finish, which means that design workmanship, and quality of finish should be quite important in your considerations, as should weld quality. Look for gaps and poor weld finish, as these may have some effect on the overall longevity of the bicycle. I would also consider dropout design very important, as these are integral to the frame and subject to failure in extreme situations. I have seen a few pictures from the more mountainous and hilly areas of Southern Illinois of a fellow who has had some failures on aluminum frames, but he is heavier rider, and some of the trails there can be rather extreme.6061 has a density of 2.70 g/cm³ (0.0975 lb/in³). 7005 has a density of 2.78 g/cm. Its ability to air temper may lead to stronger material, but it is also favored due to its greater ease of welding. Either of these aluminum choices have the proviso that finishing, craftsmanship, and care are very important in the success of the end product. Steel alloys are easier to manufacture and repair. 4130, 4140, and 4145 are alloys often used by bicycle makers. These materials can be case hardened by the presence of carbon monoxides or the use of charcoal in a case, or hardening pack, which infuses these elements into the surface of the softer steel. You could harden all the way through, but this may make the steel brittle, so hardening the exterior is best, giving the best of both worlds for what the steel alloy has to offer.Craftsmanship makes a difference in weight and repair-ability. Last week, I traded in a complete Cannondale H600, weighing 24 pounds built, and 6061 aluminum. I received in exchange a Fuji Touring Series IV frame and fork, weight built 24 pounds. I would consider both bicycles to be equal in speed, but the aluminum ride is harsher. Things can be done to mitigate this, like an alloy fork, but it is still a better ride on steel. For a mountain bicycle, with an emphasis on durability, I would prefer steel, due to its more forgiving nature and repair-ability, given the stresses involved. Fracture lines along the dropout edges and stress points may also appear with age. Steel bends, aluminum can crack instead.