2G/3G vs 4G/LTE Cell Connectivity for Sensor Prototype?
2G GSM/GPRS are almost dead in the US. CDMA is still working and it would be supported atleast till 2020 becaasue it is used in M2M devices. 3G is still one of the most widely used networks and it has great coverage and it would not likely be phased out till 2024. 4G is widely deployed now and is the future. 

Coming back to this from the perspective of developing hardware projects/products and availability of module for DIY use.

2G chip-sets are very cheap (~$10) and easily available with lots of documentation. Example 2g module here

3G modules have been in the market for a little while and there are a couple of modules at a good price-point with decent documentation. Example 3g module here

4g is still bleeding edge and there are not a lot of modules available for DIY users with great documentation and at a good price. I was able to find one with good documentation here.  

There are a couple of other factors that come in play when you are working on a project. One is just the support and documentation around the hardware. These modems usually need a good library built around them which can handle most of the functionality and most of the time, the manufacturer gives just a datasheet specifying what all the functionality is and the user is supposed to build the libraries for them in the platform of their choice. Here is adafruit's library for the cellular module to show you the complexity. Also, when you are just using a few of these then the modules are just building blocks and it usually makes sense to use the modules with the best possible support around them.

Tl;dr, use a 3G module with good software support for your platform.