Bringing back memories of my days in the sheet metal shop...heh
A good nibbler is about the best. The only downside to it is that, depending on the nibbler, you typically lose up to 1/4" of metal along the line you are cutting. So, if you are tracing templates onto metal, you will want to make sure that you leave space between the patterns. The other drawback is that they tend to be a little difficult to control if you are trying to cut a little sliver off of the side of a piece of metal.
As for the shears, this is a little hard to explain, but here goes... They shears tend to leave an "uneven" cut line. It's not that they won't/can't cut along a line, it's that they tend to bend/tweak the metal along the cut line. Take this with a grain of salt, as the last shears I was around was about 8 years ago...they may have improved drastically since then. But, the shears do work pretty well when you want to cut a sliver off of the side of a piece of metal.
Depending on what all you are planning on doing, you may also want to look into getting a manual shear & brake. A manual shear is awesome for cutting wide sections of metal in straight lines with no deformation to the metal--think of it as a big ol' papercutter. And a brake is great for creating long, solid bends in metal. I've seen a few combination shear/brake in smaller flavors--2' to 3' for a couple hundred dollars...depending on how deep the pockets are...as with most tools. There are also a few brake/shear/roll machines out there that will also give you the ability to create nice rounded corners in your work.
Hope that helps a little...
Chefout
