I've used both the 8800 and the Blackjack. Unless multimedia is important to you, I'd recommend the 8800 over the Blackjack for Email/messaging.
The most annoying thing about the Blackjack is the fact that it can't keep up with fast typing. When typing fast it drops characters. This is a really bad feature for a messaging centric device.
The Blackberry UI is also a lot more convenient for Email.
The Blackjack is better for streaming media. Partly this is because of 3G, and partly 3rd party software, and partly built-in OS support.
As for 8800 vs 8830, the 8800 has built-in GPS. It is disabled on the 8830. Otherwise they are very similar. The choice there probably depends on whether VZW or AT&T meets your needs better. One advantage of the 8830 is that you get 3G speeds in the U.S.