Before comparing LED and LCD TV, it may be useful to become familiar with the terminology which is explained on the 'What is LED TV' page. Even though LED TV is essentially a form of LCD, there is still some important factors to look into when comparing an LCD TV which uses LED or CCFL backlighting.
Contrast and Black Levels:
Standard LCD TVs have a backlight which is constantly on during use. Dark areas are created by "turning" the light away from the LCD panel, resulting in a low contrast level and poor clarity in dark scenes. With LED TV the light emitting diodes are dimmed locally to create deeper blacks in specific areas of the screen, which can also allow for better detail in dark scenes.
This image (Courtesy of Sony) shows the advantage that a Dynamic RGB LED TV has over a standard CCFL LCD TV.
The newest LED TVs can offer dyanmic contrast ratios of up to 2,000,000:1
In this case, LED is able to create much higher contrast ratios and is a clear winner between the two.
An LED TV using white Edge-LED technology for the backlight do not vary hugely from standard LCD TV. The main advantage of Edge-LED technology is that it allows for the TV to become extremely thin. The Sony ZX1 uses Edge-LED meaning it is only 9.9mm thick. However, if purchasing a LED TV using Dynamic RGB-LED such as the Sony Bravia X4500, the improvement in colour is significant. An LED TV using RGB pips LCD here, but the two are fairly even for white Edge-LED.
Traditional LCDs have always suffered from contrast degradation when viewed from angles of more than approximately 30 degrees from off centre. Although there have been vast improvments over the last few years, LCD still falls short of Plasma TVs when it comes to viewing angle. Viewing angle is determined by the LCD Panel itself and not the backlight. Therefore, LED backlighting offer no improvment in this area.
Size and cost:
LED TV using Edge-LEDs are typically thinner than LCD TVs using CCFL backlights. LEDs positioned behind the panel do not offer such great rewards in terms of space saving. The screen size range however is smaller than what is currently available in LCD. LED offer around 46"-70", while LCD offers everything from around 15" to 65". The main problem with LED is currently the cost, which is currently around twice the price of LCD . The key reason why these LED TVs are commanding a premium is primarily due to product positioning. They are currently marketed as high-end panels featuring the latest bells and whistles including Wi-Fi, IDTV and Web interactivity. Another factor is their ultraslim proposition which unfortunately comes at a price as far as aesthetics is concerned.
A few other factors are longevity and power consumption. Although there is little history to support LED TV, LEDs should degrade less over time than standard LCD, with most manufacturers claiming around 100,00 hours for the lifespan. In terms of power consumption there have been mixed reports, with some claiming up to a 40% power saving, whilst others report power consumption on a par with plasma. Without further evidence on these matters, I would have to class LED and LCD even.