A plasma cutter throws a long hot stream of molten metal out of the cut, and I would not use one for trimming a fender, especially while the fender was still on the vehicle. Plasma has a tendency to make a puddled edge on sheet metal because there is so little mass to carry the heat away from the cut, so the edge of the cut tends to melt, and without a jig or robot, it is almost impossible to make a straight cut with a plasma cutter.
The air chisel blade shown does essentially the same job as the shear, although I prefer to use an electric version of the tool (shear) rather than air. I have and have used both versions of the shear, but find the electric has more torque and is easier to control. I used the shear extensively in fabricating a new floor for my CJ-5. Due to the shape of the tools, I would expect both the shear and the chisel to turn about the same radius corner, and both have the problem of turning up a long curl of metal as they cut. I would expect the air chisel to take more practice to get a good clean cut in light metal because they stroke toward the work, and as they speed up and slow down (just as you start and stop the cut) they can hammer the work site and leave you with occasional bends in the sheet metal. The shear takes far less skill and is easier to master. For going slow and careful, the electric shear is far easier to use because it cuts cleanly and with good torque even at slow motor speeds, whereas the air shear does not run well at low speeds and tends to just bleed off air rather than reciprocate as it should.
For doing curved cuts, the shear or chisel will do a better job than a cut-off wheel, and in the end it may take more than one tool to finish the job, such as at joint lines and in tight spaces. Trying to use a straight cutting tool to make a curved line will usually lead to little jags and deviations. Trying to use a shear or chisel to cut a straight line has the same problem the other way around, they only cut as straight a line as your hand and eye can control.
Another option would be a nibbler. They can also do nice clean cuts in sheet metal without rippling the work, and they are fairly easy to control. A nibbler will cost about the same as an air hammer and chisel tool, or a shear.
A chisel, shear, or nibbler needs a clean edge to start on. The cut off wheel can start anywhere and burn its way through the metal to begin a cut. So again, it might take more than one tool to get the job done. If you only have one tool, the cut off wheel is probably a bit more versatile because of the ability to cut anywhere without a starter hole or open edge.
