User blog comment:JuneReil123/RIOT PLEASE! BRING BACK SUPPORT ZILEAN!/@comment-5924090-20130522220117/@comment-5556439-20130528101324

Nasus' is better for competitive reasons as it lowers AS and increases as the duration goes on, so you have more usage out of one cooldown. Its more cost effective and can be more effective against a higher amount of champions.

Zilean cant cross-out an ADC for 4 seconds in a fight, but he can revive one or save one.

Nasus cant boost an ADC or save them, but he can tank one and make it useless for a moment.

Considering he can peel with Gauntlet and Randys Omen but ALSO act as a meatshield and debuffer for his team makes him 2x as viable. But Zil has more Sw4G*, so Nasus can kiss his beard.