I'm in the same boat, switching from a Bold to 8900. Just wanted to throw out my likes/dislike too.
BATTERY: The 8900 is WAAAYYYYY better in this one, hands down, end of story!
SCREEN: The Bold screen is bigger, but honestly I do not really notice it. Plus the pixel density on the 8900 seems to make up for it because it looks even better than the Bold, and I didn't think that would ever be possible!
KEYBOARD: This is tough. I was worried about the Bold keyboard coming from an 8310, but I LOVED it. Switching back to the 8900 was a little strange. I think I actually like the Bold a little better for feel because they are kind of squishy, but for longevity, I can type longer on the 8900 without having my fingers hurt. On the Bold after a while my pinky fingers would almost cramp up or something from holding it up I guess from the weight or something because it is definately a heavy phone.
SIZE/WEIGHT: DEFINATELY 8900! It honestly feels like half the weight and size, but without any compromise in quality.
CONNECTIVITY: I am on T-Mobile so the 3G never effected me. Both phones seem to get the same signal and call quality, and both worked well on wi-fi.
SPEED: Both phones seem wicked fast. The 8900 actually seems faster when loading your application list, but slows down if you open a folder with stuff in it for some reason. And overall the 8900 actually seems speedier, but that could be due to the OS builds because every time I updated the Bold it seemed to help. The big point here is that I am not constantly seeing the hourlgass like my 8310!
SPEAKERS: Bold wins here. The Bold's speakers are just amazing for a phone! I would get people going, "Wow thats your phone?" all the time with that thing. However, the 8900 is still good, and better than the 8310, but just not Bold good. I LOVED how the speakers on the Bold are on the side of the device so they wouldn't get muffled when laid on a table. The 8900 speaker is on the back. Even still, the 8900 is still loud and clear, just doesn't have the depth the Bold's speakers did.
CAMERA: 8900! The best way to describe the 8900 is: WOW. I am constantly amazed at the quality of this camera. I don't even want to use my point-and-shoot because I want to put the 8900 to good use! The autofocus is WAY faster than my friends G1 and works wonderfully. Up close is amazing. I can't say enough about it.
I was worried about the switch but since I did it, I haven't looked back. The 8900 is definately better for me than the Bold.