Yeah, work properly and as expected.
Every Android phone I have ever been handed can't even get scrolling right. When I see an Android phone scroll as smooth and fluid as an iPhone, then we'll talk.
I've been a die-hard iPhone user since the original. I've gotten into TONS of arguments about how the iPhone bones all others. I've tested many different Android phones and felt the same thing you mentioned about how they aren't as fluid.
I played with the Samsung Galaxy S2 yesterday, and I have to admit, it's just as fluid as the iPhone.
As far as the original question, both OSs are capable of doing the same things, really. Every big app that I have on my iPhone seems to be available on Android phones now, too. Fluidity in the motion of things happening on the phone is finally on par with the iPhone (at least with the Galaxy S2 it is), every phone I've played with functions VERY SIMILARLY, seeing that every phone that exists these days bases their functionality on the iPhone's.
The iPhone has a better UI (to me) and it works perfectly with Macs.
Android phones have widgets (a HUGE plus to me), and have a very large variety of shapes and sizes of phones, screens, etc...obviously way more options as far as the physical phone goes.
Those are the big things that I've noticed. If Apple doesn't release a new iPhone 5 with a bigger screen, I'll probably end up switching to the S2 for a while.