They're all a combination of cerium, iron, and magnesium (perhaps some other stuff as well). Those that are higher in iron and lower in magnesium throw a lot more sparks and more easily. They are harder, more durable, and last longer. Those lower in iron and higher in magnesium are more difficult to spark, but what flies (little globs of burning metallic Mg) is much more likely to ignite difficult tinder. The downside is that they are softer and don't last as long.
If you're talking about the blocks of Mg metal, that's essentially making little chips of Mg to ignite, which in turn ignite other tinder that is more difficult to light. Once you get the little pieces of Mg burning, they're the same as the globs that fly from a high-Mg ferro rod.
And none of them are good indoor toys.
