Interesting question, I've wondered the same - caveat is that um I'm still pretty low on the ladder, so I'm greedily eyeing where you are hehehe
But as for my .02 SP, I think 100% powering up from your posts may be most valuable; because 50/50 gives you SBD but SBD by itself just sits there, but if you have a lot of SP, then it builds upon itself - you have more voting power, it's a sort of "interest". Then you can delegate SP or do other fun things.
I've even considered converting my other crypto into SP since most other crypto, if I go with the HODL life.... well, it doesn't do much for me, and if I were to swing trade I'd probably swing trade down...