Steem and SBD are two separate altoins, when you choose 100% you don't get any SBD. When you choose 50/50 you get half steem and half SBD. But! Steemit pays out SBD like it is worth 1$! It isn't! It is worth MORE! IF! If you sell it at the exchanges.
No need to pay premium. Sell SBD for premium!
OK will go with the 50/50 and watch your blog for further exchange tips Buzz.
MOre info here
https://steemit.com/steem/@bacchist/psa-don-t-select-100-sp-rewards
Good luck!