There are many DX vs Fascinate pros and cons in this section if you do a search.
I had the Fascinate for a few days and swapped for the DX. The GPS and reception were the main problems for me and they were major problems where I work, the X has no problem with either, other than that, the Fascinate is a great phone. My husband got a Fascinate at the same time and he loves his.
After using the X and then playing with his Fascinate, the SF colors, which at first seemed gorgeous and vibrant, now seem almost cartoonishly amplified.
The battery life was not as good for me with the Fascinate as the X.
Now I have the 1930mah extended battery for the X and am on my second day of charge with moderate use, love that.
Some complain about the size and weight of the DX, to me it feels solid and well made. The Fascinate feels plastic-y and fragile after getting used to the X.
I've heard the generalization that men prefer the DX and women the Fascinate because it's a bit smaller and has more of the "cute" factor.....I'm a woman and I love my DX !
I've heard complaints about the DX camera with regard to picture quality. My first thought was, so get a camera then!! I'm not buying a phone to take pictures !!
Some people feel the SF camera , although spec-wise not as nice, is more user friendly and takes better pictures. My take after using them both is that the DX camera seems more like a real digital camera than a typical phone camera. Take some time to read up on how to use it. I've taken some fantastic pictures. It really does help if you depress the camera button slightly to let it focus and then push it down, otherwise, you will be taking a LOT of out of focus pics.
I think one of the reasons there is so much debate is that they are both fantastic phones and there is no clear winner. Play with both and see what works best for you.