At first glance, they seem similar because of their large glass screens and the Android 2.1 OS, but the Droid X is technically superior. While both feature an eight megapixel camera and the ability to record video, the Droid X records video in HD and has an HDMI output. The Droid X's screen is larger, too; the 4.3-inch touchscreen with 854x480 pixel display beats the Incredible's 3.7-inch, 800x480px screen. Both phones come with 8GB of internal memory, but the Droid X supports up to 32 additional GB of removable Micro SD storage for a total of 40GB; the Incredible only supports 16GB of additional storage to total 24GB. And while the Droid X will ship with Android 2.1, the upgrade to 2.2 will become available later this Summer for all Android handsets.
With a technically superior phone offered at the same price, my guess is that the price on the Incredible will eventually drop. It's not a bad phone — as evidenced by its sales record — but for high-quality video or apps requiring more memory, the Droid X is a better bet.
Update: New info has been added!