LCDs are cheap, energy efficient, fairly space efficient (thanks to LED backlights taking up less space than the old CCFL backlights), and they have become pretty good job at color reproduction over the years. They’re not so good with dark/black colors (because they have a white backlight shining through the dark pixel).
OLEDs are more energy efficient, more space efficient (no backlight needed!), and have excellent color reproduction, especially with dark/black colors (if the pixel needs to be black, it just turns off). They can be susceptible to burn-in. They’re also extremely expensive… for now.
Read also:How does an individual LCD pixel work?
Basically, in an LCD the liquid crystal panel acts as a sort of “light valve,” either passing or blocking light from a backlight assembly (usually based on LEDs), and gating the light through an array of RGB color filters in order to produce full color images.
OLEDs (Organic Light-Emitting Diodes) are just what you’d expect – this technology actually produces light at each pixel location, using tiny LEDs made from thin organic films. They may produce red, green, and blue light directly, or alternatively may use a “white” OLED emitter and then filter that light in a manner similar to the LCDs.
Generally, OLEDs provide better viewing angle performance and higher contrast, but these advantages may be minimal compared to state-of-the-art LCDs in typical viewing conditions. OLEDs may also have shorter lifetimes than LCDs, although this is certainly not a given; there’s just not as much experience in the market with this technology.
So it’s very difficult to give a blanket recommendation as to which is “better” without knowing the specific needs/priorities of a given customer, or the particular products in question. Both can provide some extremely good viewing experiences, and other features outside the display may swing the choice between any two given products.