Easy.
Pros:
Galaxy has Android, endless customization, freedom to do whatever they want.
Samsung is a great company with high-quality electronics, and has developed many contributions to the world of such.
Even if your phone goes to hell in a handbasket, ESPECIALLY with a Samsung device, it is almost always recoverable via the software utilities available.
Samsung is very developer, user, and hacker friendly. Their software is simple.
Cons:
The only con about the non-dev SGS3 is that it has a lot of apps pre-installed, however it is not slow at all and gives those who like sharing media tons of ways to do so.
iphone is stuck just about the way it is. Even if you do jailbreak it and change the way stuff looks, it is all just about identical to how it was when it started, albeit with different colors/graphics/sounds. No advanced backup/restore features that I am aware of. No fixing yourself if something goes wrong. Drop it once and it's done (most-of-the-time).
apple is morally bankrupt and doesn't design anything. They steal others' ideas, patent them, then sue the creator for IP ownership.
apple support is terrible, they charge too much for purchase, repair, accessories, and just about everything else. The ONLY thing that is "free" is their support phone calls (if you don't count the fact that they are mentally and emotionally taxing).
In case you couldn't tell, I also Hate apple and try to steer as many people away from it as I can (from experience of working with apple devices/support vs. the rest of the world of electronics.
So, to sum it up, I recommend the SGS3 over the iphone X (insert number with small change and big price difference here).