The key feature of the frame is the ability to establish and maintain blade tension while keeping it aligned. My favorite is a Greenlee with a lever style tension mechanism.
Depending on your needs, having a continuous *********** to cut larger objects might be important, or a narrow nose to get into tight areas. You obviously can't have both at once, but arch-shaped saws are a compromise.
The Greenlee has the common option to mount the blade sideways for a flush cut, and the unusual option stick a recip blade on the end for a plunge cut. It also has dual grips, insulated cushioned handles and a knuckle guard. It doesn't have blade storage (except for the recip blade in the handle), insulated frame or glow-in-the-dark-hole coloring.