I have both and have used both. There are different pluses and minuses to each. Overall, honestly, I think it's a toss-up. A few random thoughts...
Lots of people seem to have a GPS issue with the Fascinate. I have never had a problem. Mine gets an accurate lock in <10 seconds and holds it even under tree cover. Performance is equal to the Droid X.
I like the capacitive buttons on the Fascinate better than the Droid X's physical clickies. I can navigate on the Fascinate with less effort and less worry about having a secure grip.
Outgoing audio may be a little better with the Droid X.
The continuing inability of VZW to provide a Froyo update annoys me. The biggest irony was when I went to the VZW website recently and got a big ! message on the screen saying it couldn't load because I didn't have Adobe Flash installed (because you need 2.2 or higher to install...duh, VZW).
The color balance on the two phones is very different. Video on the Fascinate is very saturated. The Droid X is more neutral, closer to actual color balance and more like what you'd see on an Apple product. I prefer that, but many people don't.
The Fascinate is just a little bit thinner and lighter. On paper, not much difference, but I carry my phone in my shirt breast pocket and the Droid X is a bigger/heavier payload carried that way.
At the end of the day, other than the Froyo issue (which you could fix by rooting), either is a very good choice. Spend some time at VZW trying both if you can.