Most LED's will give a lumen level which will help you match the brightness or existing lights. A 3watt LED may produce a light similar to a 20-30watt halogen (that is a very rough guide).
The Kelvin scale can you give you the colour temperature of the bulb (so you don't wash the campsite out with a daylight lamp with a horrible blueish tint!).
2000-3000 (approx) is warm white. 3000-4700 is cool white 4600-7000 is daylight white. Personally when camping, I cringe at anyone using anything whiter than warm white.
Also be aware that cheap lamps can make some pretty bold claims about their colour temperature and capable light levels. I have recently tested a 2.6w LED against a 4.5w LED downlight and there was no discernible difference. Both were actually cheap to buy but the 4.5w were super cheap (and dimmable).
Personally, I think the best way to choose is initially with an educated guess at what you think you need, and then use your eyes to make the final choice.