Of course, it depends on what one uses their smartphone for. But the general consensus is that Android wins out because it is more open. In other words, with iPhones, you get what Steve Jobs thinks you should get. With Android phones, you can customize the phone to look and feel however you like. Anyone can create apps and (unlike iPhone) nobody censors them.
The apps for both phone are comparable. With one noted exception (the popular "Lose It", a weight loss app for iPhone), virtually any app for the iPhone is available in Android (and Lose It is coming to Android soon anyway).
The fact that anyone can come up with apps for Android is no small matter, because many apps can cure "defects" with your Android phone. Even defects that you weren't aware of. For example, someone created an Android app called Swype, which allows a simple way of inputting text. Instead of hitting the keys on the screen one at a time, you simply glide your finger from letter to letter, and it figures out the word (even if you don't hit the keys dead on). As a result, I can now "type" text faster with one finger than I can with two fingers on a regular screen keyboard.
The only thing where iPhone excels is playing music and integration with iTunes. And that's a huge selling point for some. But Android phones integrate better with all of Google's applications, including Gmail, Calendar, Google Voice, and perhaps most importantly, Google Maps, which makes every Android phone in effect a state-of-the-art voice-activitated GPS unit.
And Android 2.2, which is being rolled out now, has one superlative feature that iPhone 4 lacks: tethering. This basically means that your Android phone operates as a mobile WiFi system. No need to look for a Starbucks for your laptop or iPad — your phone will act as the WiFi, whereever you are.
Multitasking? Android wins. Battery life? Android wins. Ability to run Flash? Android does; iPhone (Steve Jobs assures us) never will.
And Android phones tend to be cheaper, have cheaper phone plans, and you're not stuck with only AT&T.