I love both phones...I've still got both of them, but I prefer the X for a couple of reasons.
1. Build quality. I simply cannot stand the slick plastic battery cover. I refuse to use a case on my phone because I don't drop them and it just adds bulk. The slick back on the Fascinate is so annoying. It scratches just by putting it down on your desk a few times. I had to get a Invisashield for the back cover just so I wouldn't be driven nuts by the scratches. The good news is that the replacement covers are only 5 bucks from Verizon so I could replace em easily, but it also makes the back of the phone really slick.
2. A real LED on the X. Sure, noLED works and it's a decent enough substitute (sometimes better because you can modify colors, icons, etc.) but it's also something that is annoying. It's not the most stable program out there and it turns on and off by itself sometimes.
3. Screen colors and text legibility. It's been mentioned before, but small text is clearer on the X and the colors are more true. I'm not sure this is even up for debate. Sure, the SOLED is VERY black and the colors pop off the screen, but it's oversaturated and too colorful. Some people prefer that...and in fact, after using my Fascinate for a while, I was disappointed going back to it for the colors, but when I use the camera or look at pictures on the phone, I know I'm seeing what I'm supposed to see, not what an oversaturated color palette sees. Personal preference is obviously in play here.
4. GPS accuracy. It's a lot faster on the X without a doubt.
5. Real google searches by hitting the search button on the front of the phone. Pisses me off to no end that it defaults to Bing and you can't change that on the Fascinate. I hate bing.
As for the Fascinate...it does a few things better than the X.
1. The size is a lot more pocketable. The 4 inch screen is my preferred size and it's thin as hell. The hump and the extra .3 inches on the X make if feel a bit bulky.
2. Gaming...the graphics processor definitely helps. Games feel quite a bit smoother on the Fascinate even though the X is a fine gaming machine as well.
3. Scrolling...seems to me the scrolling seems to be quite a bit more accurate on the Fascinate. Not sure why this is...but it just feels better.
4. Battery life is slightly better than the X with the standard battery. Add the extended battery on the X and obviously it wins without adding bulk at all (1mm is nothing). Adding the extended on the Fascinate is out of the question, so I ultimately go with the X and the extended.
So really, both phones have their pluses and minuses...the X is just a better all around performer. GPS is a huge deal and they still havent' nailed down the issues on the Fascinate in that regard. I will probably never buy another Samsung phone because it just seems like they don't care to fix stuff like that. The Galaxy S line of phones have been out for how long now without a real fix?