well done full-LED Backlight > well done edge LED Backlight > well done CCFL Backlight > poor full-LED Backlight > poor LED edge Backlight > poor CCFL Backlight
What is always true:
You always sacrifice image quality for thinness of a monitor because either you got insufficent cooling or bad arrangement for a full-LED Backlight or the mirror system that distributes the light from the edges is crap. Hence a fat monitor is usually a sign IQ was the priority over design.
1-2 years ago most edge-LED Backlights were poor because everyone was racing to the thinnest display. Today we are not there yet that LED is always better, but there are good models out.