For most Samsung Galaxy S3 users the new Galaxy device would probably feel almost the same as the S3.
The Galaxy S4 is a little bit bigger and somewhat lighter version smartphone and of course it got a couple of new features like smart Scroll, Air View and Smart Pause that the Galaxy S3 is missing but for the average user the difference between the two might not be worth the money.
The average Samsung Galaxy S3 owner should consider waiting anther year. In that time period apps are expected to become more powerful and the Galaxy S4 would handle them better than your Galaxy S3.
Conclusion: Unless you are a huge Samsung fan and want to own the latest smartphone version you probably should wait!