For good information about how different batteries perform, check these threads on CPF:
Alkaline Battery Shoot Out (includes lithium AA & AAA) NiMh Battery Shoot Out (rechargables) 123 Battery Shoot Out (lithium) Note that battery performance is dependent on the drain (amp rate) and voltage needs of your application.
So if you have a dim LED light, you are probably looking at a low drain application and probably should look at the .5 amp rate graphs.
If you have a high powered flashlight, you want to be looking at at the 1 amp rates.
If your device requires a certain voltage to operate, you may want a battery that retains it's voltage longer, even if it doesn't have as much total power.
-john