My deciding factors would be:
Screen: The X has a slightly larger screen with a slightly higher resolution but the pixel density is about the same. The Fascinate has a smaller screen, with more vibrant colors, higher contrast, lower power consumption, and better daylight visibility. Most AMOLED displays have a pentile pixel matrix that can cause a slightly grainy appearance on the borders of high contrast colors such as text.
Feel: Some people feel the X is awkward to hold or put in a pocket due to it's strange shape, whilst some people don't like the glossy finish on the Fascinate. Both phones have good build quality and both lack any large amounts of metal components like their predecessors.
Camera: This one is really give and take. The X has a higher resolution and a mechanical shutter for very fast shutter reflex. The CMOS sensor on the Fascinate is much better in lowlight conditions and tends to have more saturated colors while the X can seem comparatively washed out. I personally prefer the Fascinates camera software over any other phone on the market, but the X's software is a marked improvement over the vanilla interface.
Memory: The X has 6 more GB of internal memory to give it a maximum total memory of 40 GB compared to the Fascinate's 34 GB. This could play an important role if you're a person who like to have have hundreds or thousands of applications on your phone. I personally rarely had more that 70-80 applications on my phone at one time and never maxed out my Droid's 256 MB of internal memory so this really isn't an issue for myself. Another point to keep in mind is that the Fascinate's SD card doesn't require the battery to be removed to swap it so it could theoretically be easier to "hot swap" memory cards in the fascinate.
Connectivity: Both phones support wireless N protocol, DLNA and HDMI (although the fascinate require a special adapter to do so), but the Fascinate also supports Bluetooth 3.0 which allows for much higher data rates, range and dramatically better power consumption over Bluetooth 2.0. If you use Bluetooth a lot this could be a deciding factor.
Performance: The Cortex A8 processor is generally considered to be two to three times faster than the over-clocked OMAP processor in the X. The GPU in the fascinate is also noticeably faster than the one in the X. Both phones however, are very fast and you'd be unlikely to notice a big difference except when really pushing the phone or in benchmarks.
Battery Life: The battery life between the two phones is hard to call, as both phones have software issues currently preventing the true maximum battery life of the device of being measured. Current;y though, their battery lives are too close to really be a deciding factor though. Expect a day of heavy usage and up to 2-4 days with light usage.
Microphones: The X has three microphones to supposedly help with noise cancellation while the Fascinate only has the typical one microphone. Some people say that multi-microphone technology is still very experimental and doesn't make much of a difference, although I personally don't know if it does or not. Both phones have above average audio quality in both calls and video recording.
In truth, both phones are great devices and you would be hard pressed to find better phones out there. I personally prefer the form factor and hardware of the Fascinate, but the X certainly has some important.
