I'll agree with what many said here. Upgrade from the Incredible to the Fascinate? Only if it's free. And even then, that means new accessories, so not free for me. But, if I were heading into the store today to make my initial purchase, it would be a tough call.
The Fascinate offers a larger, higher quality screen. It's up to you to decide if the extra bulk is manageable or not worth the trade-off. It's certainly not as big a difference as when compared to the Droid X.
Next up are the specs. They both offer a 1ghz CPU. The Snapdragon on the Incredible is about 5% faster in general tasks (IE, not enough to notice), but uses more power due to being on an older process (65nm vs. 45nm). The Hummingbird is supposedly 10-20% faster in multi-threaded apps, but the way Android multi-tasks means you'll almost never notice this difference. As far as the GPU goes, the Fascinate's is 3-4X more powerful, but games today don't really take advantage of it. You'll get less lag in some newer titles, but until Google adds GPU acceleration to the UI, this feature may not mean much. Lastly, the Incredible has 512MB of shared RAM, while the Fascinate is 512MB of separate dedicated memory (384MB for the system, 128MB for GPU). This may come in handy in the future, but is meaningless now. Android 2.1 could only officially support 256MB of RAM, so at 2.2 you might notice a difference with the number of apps that can run in the background. Again, not a big deal for today's use.
In terms of the camera, yea, the Incredible is 8MB vs. 5MP on the Fascinate. The Incredible also has a larger aperture, meaning it has the potential to take better photos. However, Samsung has leveraged their software strengths to good use. All indications are that the Fascinate takes excellent photos, while the Incredible doesn't. Also, the Fascinate takes 720p video @ 30fps. The Incredible can't approach 30 fps when using 720p.
Samsung is also putting a lot of accessory support behind the Galaxy S line, to include dedicated horizontal desk docks, car docks, etc. HTC's support in this area has been non-existent.
Overall, the logical choice is that if you're walking into the store today, the Fascinate is the winner. But I'm weird. I'd still take the Incredible mainly for HTC Sense. I just like the way it works. Sure, someday when HTC drops support I'll simply root and put some vanilla ROM on it and use Launcher Plus, at which point, both phones would be the same. Also, I like the way the phone looks, and I think that the form factor is perfect in terms of size and grip. But again, those are subjective to your tastes.