Tactical flashlights are a fantastic piece of equipment to own and carry.  They are compact, extremely bright, and can be found with a host of features such as hand-to-hand combat crenelations (sharp points), strobe modes, and colored lenses.  Originally, these lights used Xenon (or other heavy gas) bulbs, but in recent years the introduction of high-output LEDs has taken the market.

Long story short, you really should buy the LED variants at this point in the game.  Let's talk about the differences, though.

• Battery life:  The key advantage of the LED is efficiency, and that means longer battery life.  All tactical lights are very bright, by their very definition, and that means they put a great strain on the batteries.  A more efficient LED bulb will give you the same amount of light with longer battery life.
• Brightness:  LEDs have been around for decades, but for the majority of that time they could not compete against incandescent bulbs (e.g. Xenon) for brightness.  Recently, that is starting to change.  In just the last year or two, some LED lights have been introduced that rival and overtake their incandescent competitors in brightness, in the same size light.  One of my favorites is the Streamlight ProTac LED HL (High Lumen), which outputs 600 lumens, using two CR123 lithium batteries, for under $100.
• Bulb life:  LED bulbs are essentially a solid piece of plastic.  They have lifetimes in the tens, to hundreds, of thousands of hours, which means they could run continuously for decades without burning out.  LEDs are essentially impervious to impact damage - you'll destroy the electronics or housing before LED is damaged from shock.  Incandescent bulbs typically only have a couple hundreds to thousand hours of lifetime (weeks or months of continuous runtime), which may still be more than most people need.  Because of the tiny glowing-hot filament in an incandescent bulb, they are relatively susceptible to shock damage, especially when they are turned on.
• Explosion resistance:  Workers in some fields, such as natural gas, need special explosion proof flashlights.  With incandescent bulbs, this usually means a special mechanism in the head of the light that turns the light off instantaneously if the glass bulb is broken, exposing the glowing-hot element.  With LED bulbs, there is nothing glowing-hot that can be exposed to air, so the danger is severely reduced.