S3 Negatives: Plastic feel. Battery life is horrible. Had to buy an extended battery and case which makes the already huge phone Extremely Massive! I never found a comfortable spot to carry the phone. Hence the fact that I had to set it on the bumper of a truck because I couldn't change a tire with it in my pocket. I didn't like the size of the phone. It was hard to use 1 handed.
S3 Positives: Screen is really nice. Great to watch videos on. Camera is awesome. Main 2 reasons I bought the S3.
Maxx HD: Negatives: Smaller screen and not as bright. Although for me, the smaller screen is a plus. Camera isn't as good as it should be.
Maxx HD Positives: The phone is solid. When you pick it up, you know that you're holding a quality piece of machinery. The new version of MotoBlur is nice. I like the fact that I don't have to have 7 different screens. Also, there's a Fav App that you can keep as your app drawer which will only show those apps that you put it in. This way you don't have to search through all of your apps to find the one that you're looking for.
It's simple. Go into the store. Spend at least 1 hour! Play with the S3. Give it a good test. Take pictures. Then go play with the Maxx HD. Go back and forth. Keep in mind all of the things you're going to do with the phone. Also, keep in mind that if you want the S3 to last as long as the Maxx, you're going to need a bigger battery, which will make the phone a lot bigger. Bottom line, make you're own decision, and don't take the word of others like me and just purchase one.