UPDATE: DAY 2
Okay, I've spent two complete days with the Droid X now! I definitely have some more comments to make regarding my iPhone 4 vs Droid X comparison.
1) Thanks to those who recommended Launcher Pro. This has really made the Droid X more useable. After getting things exactly the way I want, I can certainly say it's a much more flexible interface than the iPhone. While the iPhone is simpler and smoother, the Droid X is more useful with the additional of customizable widgets, home screens, etc.
2) Both screens are very nice, but there definite advantages to both. The iPhone 4's screen is easier to see in bright daylight, contrast is better, and text is crisper. On the other hand, I really like the bigger screen on the X, as well as the 16x9 aspect ratio. As I said before, there definitely is no clear winner here and it remains a personal preference.
3) The camera interfaces and abilities are significantly different. The camera on the Droid X takes longer to start up and there is some very noticeable lag in the refresh rate, especially when shooting video. For example, while you are shooting video, the screen looks like it is showing a very low frame rate, but when you play them back, they are smooth. The iPhone 4 is faster to start and does not have any lag during shooting. On the other hand, the Droid has many more options for both still and video shooting that come in handy. I haven't had time to compare the actual output of both.
4) The battery life is significantly better on the iPhone 4, especially if you are using Live Wallpapers on the X. It's a double-edged sword, however, because you are trading flexibility for battery life The iP4 lasts longer because it is very limited in what you can do. The Droid X's battery life can be all over the place, depending on how you set it up. It's definitely acceptable, and I really like the Live Wallpapers, so I'll probably just end up getting the larger capacity battery for the X when it comes out next month. In this way, I'll have a larger battery and a spare if the need arises.
5) Widgets are a wonderful thing! I'm not using any of the default widgets, but those Beautiful Widgets are very nice. And being able to quickly toggle Wifi, GPS, Bluetooth, etc is definitely a convenience that the iPhone 4 cannot match. Being able to customize simple things like the data, time, and weather on the home pages is also something you can't do on the iPhone 4.
6) The free Google Navigation app included with Android is very nice. However, I've noticed that the GPS takes quite a bit longer to start up than it does on the iPhone 4. Using a navigation app on the iPhone 4, the GPS is almost immediately able to pinpoint my location and begin turn by turn directions. On the Droid, it takes sometimes a minute or two before it can start navigation, which is more in line with stand-alone GPS devices. And yes, I have all of the navigation settings turned on.
7) Voice integration on the Droid is awesome! I love being able to speak a lot of things I would normally have to type. It's especially handy for searches and text messaging. I'm sure this is something Apple is working on right now, but it probably won't be out until iOS 5 so this is a pretty significant advantage for Google.
I really don't think you can go wrong with either device! I'm just ready for a change, so I'm almost positive I'm going to stick with the Droid and return my iPhone 4. I'm ready to change carriers too, and if the iPhone ever does come to Verizon, I might considering it again. I also found out I get a better employee discount with Verizon than AT&T, so that's a nice bonus.
I can't really say at this point that one phone is overall better than the other. To me, the iPhone 4 looks sexier from the outside, but the Android OS is definitely more desirable at this point. It has quirks, and it's not as smooth or refined, but the freedom is something you really grow to love.
So as you probably noticed, I'm definitely leaning towards keeping the Droid!