From
http://www.pcworld.com/article/165240/article.html The tradeoffs between 2.4GHz and 5GHz have to do with interference (almost entirely in 2.4GHz), range, and speed, three properties that all relate to one another. The more interference, the less speed and range; the greater range you want, the less speed you can have; the greater speed you want, the more you have to mitigate interference and work closer to an access point.
Let's break down the band problems by these categories to see what you can do to affect each related property:
Interference in 2.4 GHz: The 2.4GHz band is a "junk" band--it's a frequency range that's heavily polluted. It's one of the most heavily used
industrial, scientific, and medical (ISM) bands, which have broad rules to allow equipment that needs to emit a signal to work in within its confines. ISM devices aren't used for communication, but produce electromagnetic signals as part of their operation. And those signals are interference.
The 2.4GHz band also has a host of other competing uses; 5GHz has the distinct benefit of having very little technology that makes use of the band. Reducing interference lets network work at higher speeds and greater ranges.
Let's look through common 2.4 GHz interference problems. (Also see "
Troubleshooting AirPort Interference," a 2007 article which is a little out of date on some topics, but has more general advice.)
Problem: Your home or office microwave oven. It uses the 2.4GHz band to heat water molecules, warming your food. A microwave oven contains a device that generates a roughly 1,000-watt signal oscillating 2.4 billion times a second--2.4GHz. And there's your interference. Microwaves are shielded from emissions, but that only reduces the vast majority. There are tiny leakages that, while nearly unmeasurable, are still enough to disrupt Wi-Fi, which uses very low signal levels.
Solution: Stop making popcorn already! And move your base station farther from your kitchen or breakroom. Or use the 5GHz band, which lacks all these interference sources.
Problem: Networking hardware and the 2.4GHz band. The band shares uses with common purposes, such as Bluetooth, cordless phones, baby monitors, and video cameras. Bluetooth has been harmonized and coordinated with Wi-Fi for years, but other devices aren't so pleasant. The British spectrum regulator OFCOM
just released a studythat showed
interference from competing devices, especially wireless cameras and baby monitors, appeared to be responsible for most Wi-Fi interference, at least in London.
Solution: Update your gear, switching to 5.8GHz phones or better behaved baby monitors (some use Wi-Fi directly now), or force your Wi-Fi clients to use 5 Hz, if that's an option.
Problem: Few non-overlapping channels. To make matters--and the potential for interference--worse, the 2.4GHz band is divided into overlapping channels. The more overlap, the greater the interference among networks located closely together. In the U.S., that means that only channels 1, 6, and 11 are mostly in the clear when used in proximity. In an urban area or apartment building you might see dozens of networks all trying to use the same frequencies, and all colliding off each other--Wi-Fi doesn't give one user's network preference to another--reducing throughput and sometimes causing the network to drop.
Solution: This will sound repetitive, but switching to 5GHz alleviates the channel problem. (I'll talk about
how to force newer 802.11n hardware to use 5GHz later in this article.) That's because so many more channels are available--and without any overlap--in the 5GHz band. (Channel availability varies by country, more in 5GHz than 2.4GHz;
Apple has a list.) 
Range: The big advantage 2.4GHz has over 5GHz is range. The shorter wavelengths used in the 5GHz band cannot penetrate as well through seemingly solid objects like walls, ceilings, desks, and people. The flip side is that 5GHz has different rules about how much power a base station can use to send signals.
Problem: 5GHz range seems quite small compared to 2.4GHz. You can go one room away and not get a strong 5GHz signal or see a network set to 5GHz.
Solution: In all likelihood, the base station automatically chose a low-numbered 5 GHz channel (or you chose it manually). In the U.S., the lower four available 5 GHz channels can only transmit at 5 percent of the power of the higher channels.
Apple's new base stations now automatically choose a higher-numbered channel by default if there's one available. If your station is choosing a lower channel in the 5GHz band, you may want to override that choice and choose a higher channel manually in order to get the signal strength you deserve.