My wife has a fascinate and I have the X.
The main difference for me is how easy it is to read text on the X vs the Fascinate. I use this much more than my pc now and find this very important, while the fascinate has a very beautiful screen (makes the X look like crap when side by side), it is an eye strain when reading text for a length of time, especially small print on web pages. I find that I have to scroll/zoom much more than the X. The pixels are very visable on the Fascinate, barely noticable on X.
They are both great phones but it will be up to your taste. I can't tell you which one you will like more than I can tell you that you will like Twinkies more than Zingers.
Spend some time at the vzw store and thoroughly check both devices. If the sales people bother you tell them that your contract isn't up for another year and the will leave you alone so you can spend some quality time with the phones.
Both have software quirks. Fascinate's gps is nearly useless until the fix arrives.
The 2.2 update on the X introduced some stability issues such as freezing, rebooting, music not playing, all promised to be fixed in the next update. Hopfully.
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