iOS is smoother, better integrated, better regulated, apps perform better and games don't run like turkey.
For reference tried both a Xperia Z2 and Samsung Galaxy S5 and neither could hold a candle to my iPhone 6.
I fired up my Nexus 5 the other day for the first time in months. Wow, man this thing is fast and smooth was what I thought.
But it has crappy reception and misses calls all the time and you have to turn off almost all the features to get decent battery life. I'm looking forward to see if Project Volta fixes the battery life.
Saying iOS is smoother means you really haven't experienced a Nexus 5 using the ART runtime.
Needless to say my iPhone 6 is in my pocket and I hope Apple addresses the performance and terrible Safari experience soon.