For average consumers, '3G' and '4G' are two of the most mysterious terms in the mobile technology dictionary, but they're used relentlessly to sell phones and tablets. If you're shopping for a new phone, the answer isn't clear-cut, and you shouldn't always go for the higher number. Our primer will help explain which technology to pick.

3G and 4G Explained

First things first, the "G" stands for a generation of mobile technology, installed in phones and on cellular networks. Each "G" generally requires you to get a new phone, and for networks to make expensive upgrades. The first two were analog cell phones (1G) and digital phones (2G). Then it got complicated.

Third-generation mobile networks, or 3G, came to the U.S. in 2003. With minimum consistent Internet speeds of 144Kbps, 3G was supposed to bring "mobile broadband." There are now so many varieties of 3G, though, that a "3G" connection can get you Internet speeds anywhere from 400Kbps to more than ten times that.

New generations usually bring new base technologies, more network capacity for more data per user, and the potential for better voice quality, too.

4G phones are supposed to be even faster, but that's not always the case. There are so many technologies called "4G," and so many ways to implement them, that the term is almost meaningless. The International Telecommunications Union, a standards body, tried to issue requirements to call a network 4G but they were ignored by carriers, and eventually the ITU backed down. 4G technologies include HSPA+ 21/42, WiMAX, and LTE (although some consider LTE the only true 4G of that bunch, and some people say none of them are fast enough to qualify.)

There are many different ways to implement LTE, too, so you can't assume all LTE speeds are the same. Carriers with more available radio spectrum for LTE can typically run faster networks than carriers with less spectrum, for instance.

This confusion is why we run our annual Fastest Mobile Networks story, which tests 3G and 4G networks in 30 cities nationwide. In this year's tests, we generally found that on speed alone AT&T's 4G LTE network was the fastest, followed by T-Mobile LTE, Verizon LTE, T-Mobile HSPA+, Sprint LTE, AT&T HSPA, Verizon 3G and finally Sprint 3G. As Sprint ramps up its faster "Spark" LTE network, we expect its LTE speeds to rise to competitive levels.

Would you like to know more about LTE, the global standard for 4G? Read our primer, What Is LTE? over at ExtremeTech.

When to Go For 4G

In 2013, almost everyone should have a 4G phone. Verizon now has nationwide 4G LTE coverage. T-Mobile and MetroPCS have nationwide HSPA+ 42 and growing LTE networks. AT&T has broad LTE coverage. Sprint is still building out LTE, but by next year the carrier aims to be comprehensive.

There's one thing to watch out for, though. Some carriers, such as FreedomPop, are still selling phones that run on Sprint's old WiMAX system. That system is deteriorating and will be turned off at the end of 2015. The new LTE system will only expand. So we recommend buying LTE devices from Sprint, not WiMAX devices. If possible, the devices should also support Sprint's new, faster "Spark" LTE system, not all of Sprint's LTE phones do.

If you like to surf the Web and especially stream video, 4G can be heaven. If you connect a laptop to your mobile link, 4G makes a huge difference. In general, anything involving transferring large amounts of data gets a big boost from 4G. Watch out for the data limits on your service plan, though; it's easy to use up a lot of data very quickly with 4G.

If you have a 3G phone and you've been frustrated with slow data, 4G may be the solution. 4G won't solve any dropped call problems, though, as all calls will be made over older networks until carriers switch to voice-over-LTE during the next few years.

Finally, if you want to future-proof yourself, get a 4G phone. 4G coverage is only going to get better, and that's where the carriers are spending most of their money right now. You can assume that all 4G phones also support your carrier's 3G and 2G networks as well.

Should You Even Consider 3G?

There are a few reasons you might still settle for a 3G phone.

If your phone is mostly for voice use, you have no need for 4G data. Save money and save battery life by choosing a device without the high-speed network.

If you live in an area that doesn't have 4G coverage, there's no advantage to a 4G phone. In fact, you'll have serious battery life problems if you buy an LTE phone and don't disable 4G LTE, as the radio's search for a non-existent signal will drain your battery quickly.

If you're strapped for cash and buying a phone off contract, you may have to settle for 3G to save money. In that case, make sure to get the fastest 3G phone possible. On Verizon and Sprint, you want to check that it supports "EVDO Rev A." On T-Mobile and AT&T, you want the highest class of HSPA+ possible: if not 42 or 21, then 14.4.