CCFL is still more efficient energy wise than LED (although LED becomes better each year), but behind an LCD there is a slight difference. The LCD has 3 colour filters per pixel, each letting true a tiny bit of the spectrum. With RGB LED's you can match the spectrum of the filters with the output of the 3 LED's, and that is what makes RGB led efficient for backlighting. Not LED per se. For normal room lighting CCFL still has an edge as the spectrum is more fluent and more natural than the spectrum from LED's, but for backlighting RGB LED's are perfect. This article shows the DELL XPS with RGB backlighting has the best gamut of any display:
Due too aging differences, both RGB Led and CCFL can colour cast over time. If you equip an RGB LED display with sensors though, it can adjust for this aging. This is what you see in high end Eizo screens for example.
The question for the panel type can be answered with TN. TN with a super-fine pitch, like this 16 inch full-HD panel, is not best for contrast. Normal TN desktop screens can get to 1:1000 today, but in a laptop, expect more like 1:400-500.
So that gives a rather weird situation. A laptop with a not so great panel, equipped with about the best RGB backlight available today. Nice if a display offers a 12 bit or more lookup table, but I guess this DELL hasn't. So what do you want with the enourmous gamut? Sure, Toystory 4 will look great, but for 99% of computer work, sRGB is still the most important colour space. With this RGB LED, realistic sRGB display is out of question, there is no hardware to calibrate the gamut down if you like.
If the upgrade costs much, you might want to skip it and buy a decent desktop screen with the laptop. But the RGB LED might save power compared to the white LED, due to filter optimization.