User blog comment:ThatPrimaryGuy/Thoughts on Nocturne/@comment-6061249-20140421112019

Nocturne's ultimate has longer cooldown since it has a global effect. Global effects are far too superior to give them low cooldown. And Nocturne's ult has better ganking potential since your chances to hit are (considering someone doesn't have a spell shield) 100%, you approach the target faster and the effect of global darkness, of course. The only downside is less range, but it's not really a problem.