Power Up 100% means all of your shares of the earnings goes to powering up your account with Steem Power. That means your upvote will be more powerfull. One benefit for you of Powering up 100% is that it increases your voting power, this is good to have.

Default 50% / 50% means you want half of your earnings to go to Steem Power and half to go to your money account (either Steem or Steem Dollars) that are readily available for trade or to pay for stuffs.

My advice for you is to try posting with 100% power up for a day then next day go with 50% / 50%

Good luck & Thanks for reading!