On a pure spec basis, both phones are shockingly similar. Each has a highly capable 8 megapixel camera, in addition to Wi-Fi, Bluetooth, GPS, 3G, and FM radios. Both have large touch screens and both lack a physical QWERTY keyboard.
The Droid Incredible's display measures 3.7 inches and the Droid X's measures 4.3 inches. If you're thinking bigger is better, that's not necessarily the case. The Incredible's smaller screen size makes it a smaller device -- one that is much easier to put into a pocket, for example. The Droid X's large display may look fantastic, but it results in a very large phone (some might call it too large).
The cameras are not created equal, either. Though the megapixels are the same, the Droid X has a mechanical shutter. This is rare for phones. It takes drastically sharper pictures. This will help those attempting to capture images at sporting events, for example. The Droid X also has an HDMI out port, which should make it easier to share the 720p HD video captured with it.
Each runs Android 2.1 and will eventually be upgraded to Android 2.2 with Adobe Flash Player Mobile 10.1, but there are significant differences in the software. The Droid Incredible uses HTC's Sense UI overlay. Sense makes the Incredible highly customizable by end users, who can tweak nearly every setting and application. HTC's software does a really good job of making what matters most easier to find and use. The Droid X has some of Motorola's home-grown software, but it isn't as refined as HTC's. You get a much more basic Android experience with the Droid X.
If battery life is the most critical component, the Droid X has a leg up on the Incredible. The Incredible's battery life is decent, but heavy users will find they have trouble making it to the end of the day with a charge left. The Droid X is much better at lasting an entire day on a charge, and has power left over in case you forget to charge it at night.
The Droid X offers better camera and battery performance, but has a large footprint. The Incredible is easier to carry around and has better software, but suffers from poor battery life.
Still undecided. Wait until July 15 and go to a Verizon Wireless store and spend a few moments with each.