The SGX540 GPU in the Galaxy is twice as fast as the SGX530 in the Droid X. It has a better screen, and is more pocketable. It records video at 30fps, instead of a choppy 24 and out of the box it supports more video formats including divx and 5.1 channel audio.
If you intend to game hard, you'll find the Galaxy based phones to be quite enjoyable, especially of they take advantage of the 6 axis Gyro. Its much more accurate (you can find great examples on the iPhone 4).
The bootloader is not locked down, and is already rooted with customizable roms being cooked as we speak. The Droid X on the other hand will likely just get root, but custom roms are out of the question.
That said, the Droid X feels a lot more solid that the Galaxy S models I've held. But in terms of performance and hack-ability, the Galaxy S is far superior.
If you're a "stock" type of a guy, the Droid X will provide a great experience, but if you want to hack away, make sure to pick up the Fascinate/Vibrant/Captivate/Epic
great post.