It depends on what you mean by smooth. If you're asking about app speed and switching between different app delay, etc, then you're going to vary a lot depending on the hardware of the Android Device. Some Android based devices have much weaker hardware and as a result, apps load slower or don't work quite right. With IPhones, the hardware is typically standard for that generation of phone, and so your experience with the apps will be fairly consistent. You would really have to compare a highend Android device with a latest generation IPhone.
In my experience at least with high end devices, they both have similar functionality and I haven't noticed any app speed issues with Android, but I know for a fact that lower end devices are often sold with the promise of being able to run everything, when they can't.
