Nowadays when it comes to lighting the blade, there are blades that are lit up using single or multi die LEDs and there are blades that are lit up using an LED string. The single and multi die LEDs go in the saber itself, meaning that there are no LEDs or electronics of any kind in the blade. An LED string blade means that the blade is full of LEDs. Both types of blades use materials to diffuse the light so that the blade lights up well.
Seoul P4s, Luxeon Rebels, LEDEngin 5 and 10W LEDs are all the kind that go into the saber, not the blade. Some of these are better than others in terms of brightness. One of the advantages of a LEDEngin 10W multi die LED is the ability to have a flash of light occur upon impact of the blade when using a soundboard that supports this feature.
LED string blades are the brightest and most evenly lit but are not the best for dueling as it's easy to damage the solder connections between the LEDs. However they can withstand light dueling.
Parks Sabers sabers use LED string blades and do not have sound. Also their hilts are hard to add sound to. Hasbro FX sabers (the ones that are around $120) also use LED string blades but they are not as bright or as durable as Parks but they do have sound.
In the custom saber world, currently the sabers with an LED in the hilt are the most common and what most soundboards support. However there are a few soundboards out there that are designed for LED string blades.
Basically when it comes to purchasing a saber, it boils down to whether or not you want to do a lot of dueling, what your budget is, if you want a custom design or a movie replica, if you want the blade to light up (and how brightly), if you want sound and if you want to be able to add your own custom sounds. It also comes down to if you want to build the saber yourself or pay someone to build it for you.
My advice is to go to the forums that Robiwon linked you to and read, read, and then read some more before asking any questions on those forums. Every question you can think of has already been answered, way more than once.