Even though LED TVs are essentially a form of LCD, there is still some important factors to look into when comparing the two and the benefits each technology possesses.
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 TVs the light emitting diodes are dimmed to create a deeper black, and can allow for better detail in dark scenes. In this case, LED is a clear winner between the two.
Colour:
LED TVs using white LED for the backlight do not vary hugely from standard LCD TVs. However, if purchasing a LED TV using RGB-LED, the improvement in colour is significant. LED TVs using RGB pips LCD here, but the two are fairly even for white LEDs.
Viewing Angle:
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. The use of LEDs has gone some way into correcting this issue and are almost on a level par with Plasma. In this case, LED TVs are a massive improvment over standard LCD.
Size and cost:
LED TVs with Edge-LEDs are typically thinner than LCD. 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 due to the associated manufacturing costs, which ultimately gets passed onto the customer.
Other Factors
A few other factors are longevity and power consumption. Although there is little history to support LED TVs, 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.
Overall it is clear to see LED has its advantages over LCD, but these unfortunately come at a premium, which only you will know whether you can justify!