Oh, where to begin . . . if you have space to increase section, low-density materials generally make for better structures than high-density materials, particularly if specific strength is roughly equal -- which it generally is for common engineering metals. So magnesium>aluminum>titanium>steel -- if you can increase cross section. You see in this in bicycle frames, where ultra-high-strength steel for light-weighting was pursued until bicycle frame tubes became so thin that "crippling" failure modes (local buckling of the very thin material) became a big issue. Aluminum didn't work because it wasn't stiff enough, until Gary Klein realized you could just go with larger diameter tubing in aluminum to make up for the material lack of stiffness. You couldn't do this with steel because it was already at its limit in that ratio of wall thickness to tube diameter -- if you kept weight constant with larger diameter tubing with even thinner wall thicknesses, crippling and denting issues got in your way.
As to denting, the type we've all experienced on car doors and fenders from door dings, such denting is sensitive to sheet thickness. A 3mm aluminum door panel weighs the same as a 1mm thick steel panel, but it is, in general, much less likely to get dented -- even if the steel is stronger. In the real world, a car company wants a weight saving from aluminum, so the panel isn't 3mm thick, it's perhaps 1.6mm thick, and you don't get superior dent resistance -- but it's much lighter than steel.
The gigacast structures on the Model Y are about cost and manufacturability improvements, not about weight savings, though there may be some, especially when you consider the body sealer that doesn't have to be used. They replace tens of spot-welded stamped steel components that are tack-welded together with a single near-net-shape part, giving much more precision and simpler assembly. The material properties of high-pressure-die-cast aluminum basically sucks, but Tesla has tailored an alloy that has decent elongation and better strength than some other HPDI alloys. And, again, aluminum just in and of itself is about a 1/3 the weight of steel. Minimum wall thicknesses in the gigacast parts are set by casting limitations -- I would be surprised if any part of a gigacast MY part is less than 3mm thick, much thicker than the prior steel components. And section modulus improvements are made with all the reinforcing ribs you see on the part. So you end up with a part that's perhaps even heavier than the original steel structure, made with a much less strong material, but equal or better in strength and stiffness as a part because of the design freedom of having a 3d part with ribs and increased section where needed instead of a 2d stamped construction. As for cost of repair, in either case if you damage the body structure much beyond the sacrificial, designed-to-crush end beams, a car is likely to be totaled. Oh, a very skilled auto body repair technician might be able to repair a more damaged sheet steel structure theoretically, but in practice, it's not economic.
So Sandy hitting stuff with a sledgehammer isn't particularly interesting. The thicker, ribbed aluminum structure of the gigacasting resists denting more than the thinner steel, but that says almost nothing about the load cases the parts actually are required to carry in normal use or in a crash. It's a circus trick.
