Which is simplest to use?
Well, that all depends. ;-)
If you are used to the MaxGUI "style" of coding, where you are presented with a rather long list of Functions to do everything with, then obviously you will find MaxGUI easier to use.
If you are open to learning a bit of Object Orientation, then the structure of wxMax itself should make the task of building and controlling your GUIs much more simple. For example, a wxFrame extends a wxTopLevelWindow extends a wxWindow etc... It's all pretty logical.
The event system in wxMax is very much more under *your* control. You "Connect" a callback function to a particular event, and wait for the system to call it. There's no need for long Select/Case or If/Then/Else statements. You will instead end up with a group of functions which you will generally give useful names to - OnClick, OnListItemSelect, etc.
Yes, wxMax is much larger... so at first you might feel a bit lost with the amount of functionality at your disposal. But of course, that is a big advantage, since you have all that at your finger tips. There are a few large sample apps, as well as a comprehensive tutorial to help you get to grips with it.