I was very much on the same boat as you. Had an iPhone 4, began to be interested in the S3 and Android back in July and was all set to switch to Android early this month, just wanted to wait for the iPhone 5 announcement, after that I just went and got my Galaxy S3 that same day after I left work.
Simply put, getting the S3 and switching to Android was one of the best decisions I've made on purchasing a new device in a long time.
My reasons for switching are quite similar to yours. I really wanted a bigger screen, my iPhone 4 just felt too tiny and tedious to use most of the time and seeing the sexy big screen of the S3 after I got to test it out for a bit just made me lose interest in my iPhone and feel how limiting the small screen is. I was also completely bored with iOS and the fact that Apple has not really been doing much with it as of late, whereas I kept seeing so many cool and new stuff in Android and how many things were far easier and user friendly than in iOS, Android got my interest and I started feeling more attracted to it than I was to iOS.
One of my fears about the S3 before using it was it's size, I feared that it would be too big to use comfortably but to my surprise the size is fantastic, feels great and is comfortable to use, it can also be used quite well with one hand. Grabbing my iPhone 4 (which I did to update it to iOS 6 last night) after using the S3 feels awkward, it feels so tiny and the icons look all crammed up. Jumping from a 3.5" screen to the 4.8" of the S3 was quite a leap for me but at this point, I just can't imagine going back to even the 4" screen of the iPhone 5 anymore, the 4.8" of the S3 is perfect and the screen looks fantastic, photos and videos look very sharp and vibrant in it.
Also coming from a 32GB iPhone 4, and previously a 3GS and iPod Touch of the same storage size, I needed a phone that allowed me even more storage than this since I was starting to feel that 32GB was not enough. The S3 allowed me to do just that by being able to insert an extra microSD card. The phone came with 16GB (they don't sell the model with 32GB here) and I ordered a 64GB microSD card and now have a whopping 80GB of storage on my phone. I can now store all my music, pictures and plenty of videos and still have plenty of space available. Being able to change the microSD card is a huge plus and quite cheap compared to getting a whole new iPhone with bigger storage. If I want more storage tomorrow, it is just a matter of dropping 50 or so bucks for a bigger card.
Other things like smart stay, the led notification light, direct call, the amazing camera app, the Play Store (which provides a much better experience over Apple's offering), the way Android does notification (along with the incredibly practical toggles for Wifi, GPS, Sound, etc), among many other things...yeah I'll cut it here less I end up writing a full blown review, lead me to go with the Galaxy S3 instead of the iPhone 5.
I love Apple, and I do think the iPhone is a great and solid device, but in my opinion at this point Android has taken the lead in terms of features and flexibility over iOS, even in user-friendliness Android has the edge over iOS on many fronts and Samsung has set the bar very high with the Galaxy S3 (The Galaxy Nexus and HTC One are also worth mentioning) when it comes to what a smartphone should be and frankly I think the iPhone 5, while it closes the gap in terms of specs it still doesn't manages to top the S3.
. The iPhone 5 just didnt do it for me and I am happy I have decided to make the switch and use a new OS for the first time in years
