Those are rather independent things. The 100% power up and 50/50 options are how the rewards are to be paid out to you from a post or comment. Anytime that 1 Steem Dollar is worth more that $1 USD on markets, you'll make more by using 50/50 reward payouts.

Powering down solely 'unlocks' Steem Power into Steem where 1 SP will always equal 1 Steem...so you'll always be 'breaking even' on this part.

This post got a little more in depth on why 50/50 reward payout pays more (when SBD is worth more than $1 USD): https://steemit.com/steemit-help/@sykochica/don-t-loose-money-by-setting-posts-to-100-steem-power

I hope that provided some clarification for you.