For the $40, the H-F air shear will come in handy. Your 3" cutoff wheel will probably cut a straighter line though! Nothing beats aviation snips though. Make a rough cut about 1/4" away, then make your final cut. The smaller the waste piece, the less the main metal distorts.
You can forget about a nibler cutting a straight line. It cuts little 1/2 moons. They get stuck in your shoes, and leave all sorts of cool marks...
BUT the nibblers do have their purpose!
The air saw........I never had one that I could say I liked.