The Dinger is meant to force people into making a mistake. Level turrets and grenades and watch a poor mid solo run to you and get stunned between 3 turrets. and when he leaves you have time to screw over his turret. The zhonyas is to save yourself while dealing free turret damage and waiting on cooldowns. IT MUST BE RUSHED. Eventually you will sell your doran's ring as it is only for early game hp mana regen and AP. All the things the Dinger needs.

Pros:
Tons of AP
Tons of mana Regen
Tons of pushing capabilities

Cons:
No Kill Stealing with Rockets... Till later
Must be good with grenade.
Zhonya's = only real protection