For me, you have to consider three main things...
1.
Size. I find that it's not always worth the extra size for brightness gained and run time.
2.
Brightness. Also consider bulb life, and color of light. This is why I like LED lights like the Cree Q5 or the P7 much better than Xenon.
3.
Which batteries and how much run time.
Here are a couple of my favorite lights:
The large one was a xenon that I upgraded with a drop in P7. I also changed the tail switch to an on/off clicky. This is the brightest of the 3 pictured, but not my favorite! This is the same thing here:
http://www.dealextreme.com/details.dx/sku.12761
2 X 18650's gives about 1 1/2 hrs. run time
The middle is a 4171 host modified with a Cree Q5. This is an excellent light using a choice of 2 X 3.0 batteries or 2 X 3.6 rechargeable batteries. Even though the larger P7 light is about twice as bright, I will reach for the smaller light every time!
The bottom is a very nice Q5 that can be bought as is from DX. It has a glass lens and an aluminum textured/OP reflector. This will give the same light as my 4171 mod.
http://www.dealextreme.com/details.dx/sku.14027
Again, for me, the smaller Q5 lights are more desirable.
My favorite of all, is a Q5 mod that I did on a host that uses 3 X AA batteries for a huge capacity and much longer run time for the Q5. It's only about 3/4" longer than the 4171 host, but is thicker. It is about the thickness of a size C Maglight.
http://www.laserpointerforums.com/forums/YaBB.pl?num=1194770080/80#96
But if you don't want to mod a light, I recommend the bottom light in the picture
for best size vs brightness vs run time...
Jay