Between the two, I would pick a Necro/PP mainly because it will DOMINATE the olympiad.
Personally, I would choose a SPS/ES because you get transfer pain, arcane agility which makes your cast even more INSANE, a strong end game summon with stun, wind shackle (slows enemy's attack speed), nice cubics, betray (makes enemy's summon attack it's owner, would be really good against a Necro/PP), and the seraphim buff that gives you a lower reuse in magic skills. Just make a boxed EE for powater and all that jazz. Your character will literally be insane.
It's really a preferance of playstyle though.