I think the Fascinate and the iPhone 4 are both excellent devices in their own right, and choosing a clear winner, as many have observed, depends solely on the personal needs of the end user. No reason to be a "fanboy" of either device. Just get what meeds your usage needs. I honestly was torn between getting an iPhone 4 with my latest upgrade, or getting another Fascinate, so my wife and I would both have Fascinates.
I took the time to test both devices, compare both, and research them all I could, and here's the comparison I came up with. Both devices have things in which they are superior.
iPhone 4 advantages:
1. The iPhone 4 takes better pictures and better videos. I have yet to personally test a smartphone - at least on Verizon - that bests the iPhone 4 in pic/vid quality. Fortunately the Samsung is the second best in both those things. The Thunderbolt might prove superior, as there are reports that it will have a new, improved 8MP camera rather than the current model.
2. The iPhone 4 has video output - not HDMI, but VGA at 1024x768, which is still pretty good. The USB to HDMI never materialized for the Galaxy S phones, and XDA folks still haven't gotten the TV out via 3.5 jack mod to work. The iPhone 4 uses the same adapter as the iPad to provide video to any computer monitor or compatible television. I really miss this on my Fascinate. Big time. And this is why I will not get a Thunderbolt with my other upgrade in April (looking to the LG revolution instead).
3. The iPhone screen is superior for reading text - it's noticeably sharper. The only con for some might be the smaller size of the screen, but I personally really like the sharpness, and 3.7 is close enough to 4" for me that it didn't bother me a bit.
4. Front facing camera.
5. The app library is still larger, and there are still big name apps, and especially games, that have not yet made it to the android market.
6. Resale value (tony mentioned below. It's true.)
Fascinate advantages:
1. SuperAMOLED screen - more readable in the day, and readable at ANY angle. I also prefer this screen to watching videos. This is a situation where - unlike reading - amazing contrast ratios and a slightly bigger screen do matter. The colors just pop. I continue to amaze people with how good videos look on my screen.
2. Customization - what would I do without Pure Calendar, or my weather widgets? My sticky notes? I know you can get this stuff on the iPhone 4, but I found that accessing that information takes extra clicks beyond what you need in Android. Jailbreaking is nice, but the Android developers are amazing and are releasing gobs of great stuff for the Fascinate. You have but to pick your flavor if you're not afraid of rooting.
3. GPU - the GPU in the Fascinate is more powerful. 3D games, such as NFS Shift, the only one I actually got to test, are just smoother and more fluid on the Fascinate. And it's simply because of the hardware.
4. Price - the Fascinate was $50 with my 2 year upgrade. The iPhone 4 would've been $200. Honestly, it would've been a much harder choice had the iPhone 4 been $50, but it's not $150 better. The iPhone 4 is maybe $50 better, but not $150 better.
Things they tie in:
1. Form factor - I call this a tie because while the iPhone is smaller, it's also heavier. They are both very slim and pocketable, but they are both cursed by slippery surfaces. Ease of use and ergonomics are great on both devices, and they feel natural in the hand, but PLEASE use some rubberized edges or something. I drop my Fascinate all the time, and friends that have the iPhone 4 have to put rubber cases on their phones or they would drop their iPhones too. I never had the Eris or the Droid X slip out of my hands, but the Fascinate does occasionally, and the iPhone feels like it would too if I used it for as many months as I've had the Fascinate. Yes, you can add rubber cases, but it adds thickness and bulk to the device and makes it far less pocketable.
2. Reception and call quality - Both devices did great. They pass my basement test with flying colors. I never noticed any abnormal grip issues making me drop a call, no matter how much of a death grip I used, on either phone. And the two handed death grip appeared to affect the signal relatively equally on both phones, dropping it a bar or two.
3. Battery life - allow me to explain. Out of the box, the iPhone 4 wins. But if you're willing to mod your Fascinate a bit, especially using night modes when available, an using a darker wallpaper and themes, your Fascinate can really give you some good battery life. Obviously it takes a little work, which some will be willing to do, and others won't.
Those are my thoughts comparing the two. I would honestly enjoy either phone, and they both have served to advance the quality of handsets on Verizon, and encourage continued competition, as we will soon see from the likes of HTC in the Thunderbolt (which I will not consider, since I now have a desire for HDMI out) and the LG Revolution, which I have my sights set on. The LG Revolution is rumored to have a pretty clean android OS on it, without anything similar to blur or touchwiz. I hope this holds true. A clean, stock android OS (obviously it's going to come with Bing, VZ nav, and the stupid trial apps...) will likely get side developing and manufacturers updates both easier and more quickly. At least that's what I'm hoping, and definitely mirrors the trend we've seen with Android handsets so far.