I’ve not had a chance to investigate the differences between these two widgets, but I was just thinking if it were possible to select more than one option at a time from either of these widgets? I was thinking that if the user makes a selection from the list, and had the command key (or some other key) held, that it would select both the current value as well as the new value. Or might this work better if the second mouse button were pressed instead (allows one-handed control).
Or is there another way to select multiple options at the same time?