I have both the iPhone 4 and the Droid X. They are each very nice in their own way.
Personally I greatly prefer the Droid X with it's larger screen for web surfing and messaging. It's so much roomier, than the small 3.5" display of the iPhone. I've had every iPhone to date, and I really like them but this time, I think Apple made a big mistake by not offering the larger size. Especially when the overall size of the phone is not that much larger. Also it's only 15g heavier, it doesn't' even feel like it either. There is so much more room when using the keyboard to reach in with a finger to scroll the screen, especially in the landscape view. The iPhone is cramped.
As far as display quality, while the iPhone specs look better on paper, in real day to day usage the difference is minimal. Too many people get hung up on specs instead of actually seeing what the phones are like. I'm platform neutral and always have a few activated smartphones.
In this case it just happened to work out that I have them both activated so I'm able to do a side by side comparison and having used them for some time now, I've developed a really liking for Android. Everyone I know that has bought a Droid X (quite a few friends) that are past iPhone lovers, are very happy with their Droids.
It's the customization too, there is just so much you can do with one, to make it uniquely your phone. Not limited as the iPhone.
Cheers...