If you're not required to 'trade in' the N900, just keep it. If you don't like the Streak you can always just use the N900 again instead. Since I got the N900 about 4 months ago, I still haven't started using it as my default phone (and until a system-wide virtual keyboard with portrait mode, qwerty and T9-like, exists, there's a decent chance it will stay that way).

I think the N900 is more than complete as a semi-netbook, rather than a phone.