[Galaxy S III General (US Carriers)] wdguy:
Here are a few BASIC tid-bits IRT the present technologies. Although I am not a cell phone or equipment expert, I am an expert at design and delivery of technologies and their systems.

4G is an upgrade of course from the 3G. Actual performance varies but the best case for 3G (Sprint/TMo) is ~3Mbps and actual just below 1Mbps. Sprint and Tmo are largely 4G but still maintain 3G to resurrect those remote dead zones. 4G is very similar to WiMax and almost synonymous. WiMax delivers best performance at ~ 30Mbps but actually hovers around 5Mbps. (All these are general figures I use when designing systems). You might be familiar with ClearWire or CenturyLink, a 4G service. LTE is yet another upgrade to 4G capable of breaking 100Mbps, but in actuality are ~10Mbps. The best I've had with AT&T is 11.5 Mbps, with Verizon testing slightly higher. The differences in performance will vary by carrier and location because of the network they've chose to build up in a specific local and not the underlying technology. Carriers will build more into a location to obtain or maintain a targeted market segmentation and also consider future value (FV) and return on investments (ROI). This means you'll need to do a little homework and compare performances and factor in your needs and uses. All networks will excel where the others are weaker but some just have more of the stronger features due to larger build ups. If you live in a chief metropolitan area, chances are you may be surprised at the similarities between competitors. Additionally, consider the WiFi calling feature of the GS3. You can set it to use WiFi to call which is tantamount to full signal, WiFi only, WiFi preferred or Cellular Preferred. Keep in mind that if calling on a WiFi network, moving out of range will probably drop the call.I keep my setting on Cell Pref since I have unlimited minutes and will automatically switch to WiFi if no carrier or waning carrier signal is experienced. Consider the TechTiles, Samsung's NFC technology. They run about $15-$20 for a pack of 5. Utilizing the new BT technology, they are essentially data and communication protocol logic chips that can be programmed to automate many functions in your phone when it is in the vicinity. Features like sending a preset text to another device, enable or disable phone features such as WiFi, BT etc are some of the many perks in ICS. TechTiles are small stickers the size of a postage stamp and just as thin.

Verizon: Most expensive, yet probably the best performing since they essentially keep their network channels wide open to take advantage of CDMA (coding) technology. Your data/voice/text is subjected to a proprietary coding algorithm with little compression and can handle jumbo packet technology. Less parts of your message are broken into packets and require reassembly at the receiver end. This also gives the best voice clarity bc other technologies will break a message into bits and replace some of the packets with smaller compressed bits, thus giving a slightly less than original or live sound. More noticeable when compared side by side. Alas, their GS3 seems over the horizon, but they have been rated the top provider for a decade by Consumer Reports in terms of service and customer relations.

AT&T: Consistently rated the worst in customer service and stable service performance by CR. Their GS3 seems like a carrot on a stick. They do have appreciable plans and coverage and performance, but they have a longstanding propensity for poor customer service. They have been making strides to correct this but it is not one of their premier concerns. They have a solid LTE system but do not enhance remote regions, where they lease tower services from smaller partners. You will continue to see the latest technologies fruition with AT&T in larger metropolitan areas, but again, the top three providers are relatively equal in these areas and with the WiFi feature on the GS3, I consider them to be very similar for many users. GS3 is limited to 16Gb models only.

TMo: Forbes rated TMo the best value, although not performance. Their GS3 is listed and has been in stock since the 23rd (16Gb). I've yet to see a proprietary modle >16Gb on the street. There are 32Gb unlocked at several major retailers for $800. They also are the quad core international version. Since the RAM is non-upgradable, I prefer the 2Gb of RAM (US) to 2 extra cores(Int'l). Also Samsung will soon release in the US its ultra-ultra class10 (32Gb) SD card which will seriously enhance data storage and retrieval. Although Tmo GS3 is listed at $279, they have an in store instant $50 rebate. They will mandate the device insurance which can be cancelled the following day. Unlimited voice/data/text is $50/line. Tack on another $20/mo/phone bc the actual price of the phone is ~$450. $229 after rebate in store OAC, and the rest is the $20/mo tacked onto your bill for 20 mos. This is still over $100/mo cheaper than AT&T for the comparable plan of unlimited across the board. Unlimited data is considered 2Gb/Mo. Overages are automatic with AT&T at $10/Gb and Tmo will only titrate the speed and not tack on fees. With unlimited minutes, mobile to mobile calling, A-List and rollover minutes have little use. Although TMo ranks third in the US, they are larger than AT&T and Verizon combined. According to the CIA tracking facts, the US avgs 1.5 cellphones/pp while UK has 3.5/pp. Odd since I find more than 1 a hindrance, but their networks are faster and different beginning from IPV6.

Sprint: Since they are still in limbo on their network and services, comparisons at this point would be scalable, but again probably rival in large areas and have the most room for improvement of all the providers.

In conclusion, since there are so many advances in technology, albeit relatively bottlenecked recently, it would be prudent to not only weigh the carrier's service but also consider the device features. You could be paying much more for the same or less service. Now is the time to contrast your options before setting a lenghty contract. Once last note: AT&T and TMo phones are cross network capable. The provider must release the device ID (unlock) to allow for use on another spectrum. Your device will probably need to be OFF contract, and acct in good standing, but they will unlock it.