When should I use a rounded balloon whisk attachment vs a bulb-shaped balloon whisk attachment

This is a little speculative, but I'd use the taller one for larger depths, and the more spherical one for shallower work.

You could use the elongated one for everything, except if you have a small amount of something stiff, when the longer wires might flex too much. The shorter one might splash a little less in a shallow container as well

But I've never had both for the same mixer - my Kenwood stand mixer has a different shape again, closer to the ball shape but tapered at the bottom, to better follow the shape of the bowl, while handheld ones have always been the tall sort.