Better ones have sturdy frames and use levers rather than wing nuts to tighten the blade. Some can hold a blade at 45 degrees to facilitate flush cutting. Look for bimetal blades. Better still, choose a knife that doesn't require a screwdriver for blade changes. Quality putty knives and taping knives are springy and flexible and hence should not be used as scrapers.
Scrapers are much stiffer.
These knives look similar, but they do different things. The 12-inch blade lays up the final coats and helps achieve a smooth, flat surface. Mud Pan A mud pan holds joint compound when you're taping and smoothing joints. It's also handy when you patch walls, which everyone has to do sooner or later. The straight edges of the pan provide a surface to clean your knife on between strokes.