LED's without a doubt. Neon requires a power invertor that can actually be dangerous. It converts 12 volts DC to several thousand volts AC. Trust me...I grabbed the loose end of one of the wires, you'll think you were shot. Anyhow, LED's don't draw any where near the ammount of power as neon, so you could leave them on for a long time, plus the compurer control..and many other advantages.
Voltage doesn't matter. Its the current (amps) that shock you. I = V/R (Current = Voltage/Resistance). You could have 10,000 volts at 0.5 amps and you'd be fine.
LEDS are brighter, from what i've heard.
Good for underbody kits, however I don't know how durable they are. Careful on speed bumps.