Not sure if this is article worthy, but with my addition of an older Emergency (just 121 frequency) wanted to find out more about it. NOAA has some great info on both the 121 and 406:

ELTs were the first emergency beacons developed and most U.S. civil aircraft are required to carry them. ELTs were intended for use on the 121.5 MHz frequency to alert aircraft flying overhead. Obviously, a major limitation to these is that another aircraft must be within range and listening to 121.5 MHz to receive the signal. One of the reasons the Cospas-Sarsat system was developed was to provide a better receiving source for these signals. Another reason was to provide location data for each activation(something that overflying aircraft were unable to do).
Different types of ELTs are currently in use. There are approximately 170,000 of the older generation 121.5 MHz ELTs in service. Unfortunately, these have proven to be highly ineffective. They have a 97% false alarm rate, activate properly in only 12% of crashes, and provide no identification data. In order to fix this problem 406 MHz ELTs were developed to work specifically with the Cospas-Sarsat system. These ELTs dramatically reduce the false alert impact on SAR resources, have a higher accident survivability success rate, and decrease the time required to reach accident victims by an average of 6 hours.
Presently, most aircraft operators are mandated to carry an ELT and have the option to choose between either a 121.5 MHz ELT or a 406 MHz ELT. The Federal Aviation Administration has studied the issue of mandating carriage of 406 MHz ELTs. The study indicates that 134 extra lives and millions of dollars in SAR resources could be saved per year. The only problem is that 406 MHz ELTs currently cost about $1,500 and 121.5 MHz ELTs cost around $500. It's easy to see one reason for the cost differential when you look at the numbers. However, no one can argue the importance of 406 MHz ELTs and the significant advantages they hold.
For more information on the differences between 121.5 MHz Beacons and 406 MHz beacons click to view a Comparison.
Due to the obvious advantages of 406 MHz beacons and the significant disadvantages to the older 121.5 MHz beacons, the International Cospas-Sarsat Program have made a decision to phaseout 121.5 MHz satellite alerting on February 1st, 2009. All pilots are highly encouraged both by NOAA and by the FAA to consider making the switch to 406!


They also have a great comparison of the two here:

http://www.sarsat.noaa.gov/406vs121.pdf