In 2015, it is almost always advisable to opt for a 4G capable phone over a 3G one. Most carriers have significant 4G LTE coverage that continues to expand. 4G offers superior download speed to 3G and is in almost every way superior to its predecessor. Some people who rarely use their phones for anything beyond voice calls and text messages may want to consider 3G for budget reasons. Otherwise, 4G is generally the better option.
The terms 4G and 3G are short for fourth generation and third generation, respectively. They are broad descriptions of a collection of mobile data transfer technologies based on when those technologies became available. Types of 4G include HSPA+ 21/24, WiMAX, and LTE, or Long Term Evolution, which is arguably the only true form of 4G data transfer and is quickly becoming the dominant technology.
In 2015, phones typically use 4G connections to stream music and high-definition video among other popular activities. As such, any user who wants to use these types of applications even occasionally should choose a 4G phone. Additionally, nearly all 4G-capable phones are backwards-compatible to work with 3G and 2G infrastructure.
Some users may prefer 3G if they have little interest in data-heavy phone use and want to save money and battery life. Additionally, people who live in areas without 4G coverage may prefer 3G as there is no advantage to the newer generation without the supporting infrastructure.