50/50 is this most profitable option right now, because of the increased value of Steem Backed Dollars.
Powering up your posts gives your rewards in Steem Power, while 50/50 gives you half Steem Power, half Steem Backed Dollars. However, right now, it's more profitable to go for a 50/50 split, then sell your SBD for Steem, then power up to Steem Power, because 1 SBD is worth much more than $1. (Although you may want to keep your SBD, instead of powering up)