I don't think it makes alot of sense for a person with an S3 to upgrade to an S4. Obviously if I got one for free I'd take it... but I wouldn't pay any significant amount of money for it. The S4 is basically an improved S3... is it better? Yes... but not enough to spend a bunch of money.
The Galaxy S4 is more for people looking to upgrade from the Galaxy S2 or phones from that generation or older.
I don't think it makes alot of sense for a person with an S3 to upgrade to an S4. Obviously if I got one for free I'd take it... but I wouldn't pay any significant amount of money for it. The S4 is basically an improved S3... is it better? Yes... but not enough to spend a bunch of money.
The Galaxy S4 is more for people looking to upgrade from the Galaxy S2 or phones from that generation or older.