In terms of web browsing, there isn't much of a difference on both handsets. The N900 still has some advantages in terms of the full desktop web experience. Flash compatibility and page rendering are two of them. But the Eclair browser isn't too far behind. It is certainly fast. So both are very comparable. In certain cases, I've found Eclair to be faster than the N900, probably due to a higher speed processor. Coming from the N900, I was skeptical of Android's browsing experiencing but it hasn't disappointed me yet.

As for the Email experience goes, the Gmail integration on the Vibrant is great. That's an Android thing I believe. Notifications are handled really nicely. Emails open up instantaneously and attachments are handled well. I'm using the Samsung mail client for a Hotmail mailbox and it does the job. In some cases, the Gmail app functioned better than the N900, because I found modest to be clunky when it came to handling huge files. I emailed about 5 pictures close to 6MBs today and it happened pretty painlessly from the Gmail client.

Notifications are a nice experience so far. Not as intuitive as the N900 but pretty good nonetheless. I used eBuddy yesterday for my IM needs and it was great, sans the lack of a physical qwerty keyboard of course. I'd say, go for the Vibrant because it's newer, has a longer shelf life left and it's always good to try out a platform you havn't used. You can always sell it back and buy an N900, and actually save some money in the process.