Light gathering in M43 vs FF

Most of time you will not get the same image with different cameras, because the lenses have different bokeh; because one lens can't be open as wide, or closed as narrow as the other; because the efficiency of the sensors is not strictly proportional to the size. It's a moving target, and there is really no reason to make two images with different cameras that are exactly same. Simply look at the samples for the kind of use that you have in mind, and decide if a given camera gives you the result you want at the cost that you can afford.

Vlad