Well when i played syndra i build according to my enemy:
AP Enemy: Grail/Abyss(Fed lb?)
AD Enemy: Tear /morrello(&Swain) /zhonays
Example of my build is : Grail>DCAP>Void>Zhonays>(Last item is situation ) : Ryalies/GA/Banshe/Abyssal/Liandays/Dfg
Was never a real fan of dfg, i feel its not worth, and a over kill. Also the damage over time from dcap out shines dfg early/mid.
I dont think i ever bought abyssal, only bought lindays once for a mundo, never try'd Lucidity boots seeing how she has really low cd's already(Maybe its good after blue pot not giving CD anymore)
If You specificly looking for Tear Or Morrello - Morrello is super strong atm - High ap base+cd+regen, and tear Takes FOREVER to finish.
Tear does give u like +40 ap late game and a decent shield, but weakens your strong early+mid game, and lose out on the CD.
http://na.op.gg/summoner/userName=cutrate <- Me . Dont have ALOT of games on her, but a decent amount :) Havent played her too much since pre-season started, trying to pick up new champs and increase my champ pool