For aluminum, like was said, a jigsaw is far too powerful, whereas a dremel is powered just right. For steel a dremel is not powerful enough, whereas a jigsaw is powered just right. Imagine using a chainsaw to cut a loaf of bread, it will be all shredded and mangled. Now imagine cutting through a nice thick tree with that same chainsaw, the application that it was designed for. It cuts through nice and clean like, going at a very acceptable pace. Now use a breadknife to cut the bread, will do it nicely and cleanly. Use that same breadknife for the tree. Will take FOREVER, and will look horribly, and you may go through several blades, but you will eventually get there.
To be short:
Aluminum cutting = Dremel
Steel cutting = Jigsaw