as always both have their advantages and disadvantages. S/S gets higher def but sacrifices a bit of damage with the sword. S/S Also gets the block factor which will save your life in PvP.
Spear hits harder with a pure int build but you sacrifice defense for damage. Then again you get the passive HP skills too and that mana shield skill.
Personally I prefer the S/S Nuker. Blocks have saved my life more times than I can count. Get urself a shield with a decent BR and you'll be a pain to kill. (I know I am with my BR 19 shield+5

) Not to say your not gona die alot. Thats just a fact that as a nuker your gona get killed alot. I would say S/S is more effective overall though.