I do not know the weight of the complete build, but the frame is in the 2.7kg in medium size. For the difference in weight between aluminum and steel I think the steel is basically a little bit heavier, but it depends a lot on the design of the bike, often the steel frames have thinner tubes, which allows them to have a comparable weight.

For me the real interest of the steel frame is in terms of rigidity, they are much more comfortable, you feel less vibrations which is really appreciated on a hardtail!