I work for the T and the majority of my day is handling manager escalations from irate customers, the majority being Iphone
The S3 can be had for $99 dollars at Radio Shack, and Walmart had it for several weeks at at .92 cents/
Also a refurbed Iphone 5 is $149 and a refurbed S3 on our site is at $29. Huge difference.
So, you can still get an S3 cheaper than an Iphone 5. And the Walmart price is $127 on the phone, and the huge lure is you can finance it and pay Walmart and Apple even more money.
As for the Mobile Share plan, your math is a bit off. Four smart phones with ten gigs of data has a base cost of $240 dollars. With Tax that will run you up to $280.
I personally use T-Mobile because I get free wifi calling in the US and Outside of the US and it does not use my plan minutes, unlimited data that only slows down at 10 gigs, and unlimited international text, for 95 a month, things I could not do with my own employer. That and I like to keep personal items apart from work.
I use a Galaxy S2 skyrocket that has been doing all the things the iphone 5 for the last year, and has a much larger screen, and i have 80 gigs of memory in it, and the only cost I paid was 30 dollars for the sd card.