The strength of steel vs the price is a better value for steel. But the downside is steel is much heavier.
For sliders, definitely steel. For skids, probably depends how much you plan to use them, steel is a lot more rugged. If they will get constant abuse, I just don't think aluminum would hold up well. If they aren't going to get much use but you want light weight, aluminum should be ok, or maybe looking at being creative and having plastic(Like cutting board material) skids made. Only catch is they would need some kinds of steel support structure, using plastic as the sliding wear surface(can't be bent and welded like metal, so would need a support that makes the bottom of the x flat).