If you go power up 100% all the rewards will be in Steem Power, wich affects youre voting power, the more Steem Power you have, the "stronger" you are in the community.
Default, will give you 50% SBD (steem dollars) and 50% steem power, steem dollars can be traded on the markets!
If you decline, you get no rewards.

The FAQ section explains it nicely, go check it out brother ;)
Peace, Carlos