It very much depends on the context and on what you really want to make out of your rewards. In the beginning, if you want to get to a point where you have access to more features (like sliding bar for upsteem) then you should go 100% Steem Power. But if you need liquidity, to take advantage of market spikes, then 50/50 is probably the best.

At the moment I'm 50/50 but this could obviously change.