We are talking rewards:
A) Power up 100% = saving and building your Steem Power and therefore upvote value. For long term. Post will pay out 100% in SP only, no SBD. You really don't care about short term gains, you want long term to build an income from your Steem blog, use this to power yourself forwards in the beginning (and later, whenever you feel like). It's all part of the game :)
B) Default 50/50% = you don't get to adjust this until you have much more steem power. You will get half in $SBD half in SP. Best for most everyone.
C) Decline Payout = Whales only, generally speaking. Really you did it for the greater good of steem community, or it's a post about something that paid in other ways, or commercial, or simply any reason politically it would be wrong to take payout.