I have the S4. I love the screen and how light/thin the phone is. Of course the materials probably don't appear as high quality as the Maxx.
I think the camera on the S4 is good (not in low light though) I think this is a con on the Maxx.
As others have mentioned TouchWiz sucks. It just doesn't add a lot of value considering the power it takes to run the phone. It is inefficient and takes up most storage on the S4. I often get an error when using Spotify or Google Music which is annoying.
I haven't used the Msxx do I can't say which is better. It depends on what is important to you.