The X and D2 are definitely not the same phone. They don't feel like the same phone and don't handle like the same phone and don't sit in your pocket like the same phone. Form factor is important.
Fascinate has a more open architecture (no locked down bootloader, so the kernel can be replaced). Smaller form factor and screen. I think 4" is about the perfect size for a phone screen, but I don't like the AMOLED screen. It looks grainy to me (the pentile matrix is distracting and I like to read a lot on my screen), and the unnaturally saturated colors aren't really nice for anything except the app icons. Plus the GPS and cell radios are weaker than the ones in the X. On the other hand, the audio and camera quality are much better than the X.
The X has better build quality, no stupid slippery back (for the love of God, why do cell manufacturers insist on building things that we hold in our hands to be made of hard, glossy, slippery plastic?), noise cancelling mics, great cell and GPS radios, and a bigger screen that displays text better and doesn't have heavy-handed saturation. Those things make it a win for me, but if you value smaller size, camera quality and music audio quality (NOT cell phone call quality) - or if it's a deal breaker that the X has a locked bootloader - then the Fascinate is better.