I've been wanting to do a quick compare/contrast between the Eris and Incredible, and this seems like a good place to do it...
Now that I've had a few days to play with both (my wife inherited my Eris), I'd have to say that, far and away, the main difference is speed. I was happy with the speed of the Eris - until I got the Incredible. Now the Eris is extremely frustrating.
My main piece of advice if you're happy wth the Eris is this: don't touch the Incredible. The experience is analagous to moving from a toaster oven to a microwave; the toaster oven is awesome - if it's all you know - but once you use a microwave, you'll wonder how you ever survived without it (actually, that might be a bad analogy... I wonder how many people on this forum have ever experienced life without a microwave). Anyhow...
Yeah, the UI and browser are FAST.
Other than that, the screen is bigger, and sharper - sort of. Edges of objects and text on the Inc's AMOLED screen tend to be a bit fuzzy if you look really close (not that you often would). The Inc's screen is more vivid, but doesn't have as much black/white contrast as the Eris.
The compass on the Incredible seems a lot more accurate - at least indoors - as well.
From a style perspective, I like the Eris' form-factor a little better.
The onboard memory on the Inc is a great place for pictures, which load faster than from an SD card (sure, you could store them on internal storage on the Eris - but I like having them seperated)
The Inc already has a leg up on the Eris from an OEM accesory standpoint, with a pretty nice car dock (my only gripes being that the power cord still plugs into the phone, versus being a true dock system, and that the phone doesn't auto-detect car mode)
The optical joystick is a step up from the trackball - but still just about as useless from a practical standpoint (and it doesn't light up for notifications, which is mainly what I used the trackball for).
The camera may be 8MP, but I don't find that it performs much better than the Eris (same sensor size, I assume). The camera on the Inc, like everything else, is faster, though.
Overall, the speed difference and a few other tweaks (like the dual-LED flash, which may make a better flashlight than camera flash - once someone writes a working flashlight app) make me prefer the Incredible over the Eris. Having the official version of 2.1/Sense is also really nice.
In summary: if you have the Eris and like it, you probably don't need to spend money on the Incredible. Just don't play with the Incredible; you'll probably decide you need it...
If anyone has any specific questions related to one phone versus the other, let me know - like I said, I have both, and both are operational. The Eris is non-rooted, and on 2.1v2.