What's the difference between aluminum and aluminized baking sheets?

Aluminized bakeware is made by coating another metal, in this case steel, with an aluminum alloy. Aluminum bakeware is generally stamped and cut from sheets of actual hardened aluminum. The aluminized pans/sheets are supposed to be sturdier than regular aluminum sheets, and less likely to warp or dent. They also claim to heat up faster, and could develop hot spots. But I think that can also be influenced by how thick- or heavy gauge, the aluminum is. If it's a thin pan, regardless of what it's made of, it's going to be more likely to warp, dent, or bend. At any rate, you buy what you can afford, and upgrade as your use demands and budget allows. And even then, you keep the thin pans, because they work just fine to catch overflow from a casserole or pie. Use over time will tell you if one style of your pans bake/roast faster than the other.