Zhonya's Hourglass or Rod of Ages for Karthus? (Mid)

So earlier today I reached level 18 as Karthus (just him today btw) with a full build. I had Sorcerer's shoes, Rabadon's Deathcap, Archangel's staff, Rylai's crystal scepter, and the Void Staff. However I couldn't decide what I wanted last. As the title asks, which would you recommend and why?
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