Lumens? Terralux and a mini mag

I have several questions I need some input on.

First off...what is a lumen vs. candlepower?

I have a Streamlight Stinger right now. I'm sure you all know that its 15,000 cp and streamlight says its lumen rating is 125 lumens. But if you look at the 40,000 cp Stinger HP the lumen rating is 120 lumens!! Why is the lumen rating not significantly larger with the larger CP rating?

Next...

I am looking at the Terralux 5ex upgrade for my mini mag. They claim that the lumen rating is 140 lumens. But I have seen other claims that it is around 80 or 100 lumens. Can anyone shed any light on this?

So is this upgrade actually going to be brighter than my Stinger??

Lastly...if I go with this upgrade I was going to get a tailcap switch (or clicky as i've learned from here ).

Here is the clicky LINKY

Why does this simple clicky decrease the output of the LED?


Thanks. I know i've asked a lot, but I need a good light for patrol.