The plasma sabers have 2 benefits...they have the cool extending effect when you power them on, and they tend to be a little brighter...the main drawback with them is that the plasma has to be in a glass housing...making it fragile...
The EL ones are awesome, nearly indestructible (you can actually have lightsaber fights) EL has advanced to where they are nearly as bright as the plasmas, and they are ALOT cheaper on average (not to mention easier to find, tons of EL saber makers around, very few plasma makers)
Aeryk