Pros of iOS4 is that it's a million time easier to develop for. One handset, all the same spec, don't need to worry about anything. Android is harder to develop for. There's been a few apps I've bought for my HTC Desire that you can clearly see have been developed for an earlier Android phone.
I do prefer Android though, and each update it keeps getting better and better, and newer phones that are running Android are all coming out with similar specs, such as screen size etc. Especially now with Android 2.2 you're able to install apps to the SD card, bigger and better apps will start being developed.
Android is catching up with iOS4, and the iPhone is getting some serious competition now from HTC phones, especially with the Desire.