The 3GS was out before Verizon even sold any iPhone, and they started with the iPhone 4. Only AT&T has the 3GS. You would be better off just getting the iPhone 4 for $99 because the 3GS is going to be out dated within a year, so an iPhone 4 would last you much longer.
Sprint is better than T Mobile when it comes to quality. Their phones are better and run more smoothly, and their coverage/signal is way better than T Mobile. T Mobile is only good for their price, but their signal is so bad that it's not even worth it.
The iPhone 4 has a front facing camera along with the initial back camera, it has Retina Display (basically high definition graphics) it runs more smoothly than the 3GS, and it's more smooth. When it comes to storing videos and movies, either phone is okay for that, but if you have too many then...
It shouldn't cost you any money to do this. Turning off those two settings you mentioned just means that you wouldn't be able to do anything online until you turn them back on again. You can even use your Wifi without turning off those settings and the battery life should still be fine.
Hope...
Hands down, AT&T has far better signal/coverage than T Mobile. The only good thing about T Mobile is their prices, and sometimes it's not even worth it since the coverage is bad.
The performance of the phone will be the same as any other network carrier, but the coverage and signal won't be good, mainly because that's just how T Mobile's coverage is.
IPhone's run more smoothly than droids and have more apps on their market place. But Droids are more customizable. It depends on what's more important to you. They seem pretty evenly matched recently.
It's probably because Apple has no interest in having T Mobile with their iPhone, or at least not yet. If you really don't want to switch to one of the other 3 networks that provide it, there is a way to "Unlock" the AT&T iPhone so it can be used on T Mobile. You can easily find instructions on...
The Galaxy S II is an outstanding phone which is well worth the money. The 8 megapixel camera is great, the phone runs smoothly, and the large screen is good for reading and texting.