As far as i can see, there are only a few advantages in upgrading to 
the newer S4 model, the first reason is because of the improved optics of the camera, now it has a 13mp shooter compared to the s3 8 mp camera that often never performed as well in the evening when there was less light to go around. Another good reason to upgrade is because of the larger full HD 5 inch screen that gives more space for texting and for doing other tasks, the size increase from 4.8 inches to 5 inches in real terms does make that difference.
Some things have unchanged such as the amount of storage that is offered compared to the previous model which matched the iPhone with 16GB, 32GB and 64GB models plus a micro SD card slot. Also on the software front is not any reason to upgrade, because the s3 and the s4 are both running the latest Android os, which is Android 4.1.2 Jelly Bean. However the Galaxy S4 does have a few newer features that include Smart Scroll, Smart Pause and Air Gesture to name a couple. However that the Galaxy S3 will be updated with some of the new features in the near future.
The other reasons listed why you should upgrade to this model seem to be marketing gimmicks to make you buy it. For example they say that the screen 441ppi pixel density is higher than the Samsung s3 which had a ppi density of 306, but in the real world that does not mean that much, due the the human eye not being able to tell the difference between that of the Samsung s3 and s4 screens. due to the ppi density of the screens being so high on both of the models.
Often it is quoted as having a better battery than the s3, because it has been upgraded from 2,100mAh to 2,600mAh. However in real terms it will not perform much longer than the s3, due to the larger screen using more power and there being more processing power in the s4 version. For example the new Exynos 5 Octa processor has four Cortex-A15 cores clocked at 1.6GHz for performance while the remaining four are Cortex-A7s clocked at 1.2GHz for less demanding tasks and power saving. However in real terms the new Exynos 5 Octo-core processor in the s4 is likely to be prove to be more power hungry than the previous Exynos 4 Quad processor which was used in the s3 version which was clocked at 1.4GHz.
So to conclude if want a better performing camera and a larger screen size go for the upgrade otherwise it just not worth upgrading to the s4.
