A thief made off with my D3 christmas eve, the insurance company replaced it with the 900BT.
To answer your first question, the differences between the 90BT and the 900BT:
90BT has 4V rca output
90BT includes the ipod composite cable ($50 separately)
90BT has a 2 year warranty instead of the standard 1 year.
Those are the only differences.
Now....would I choose to upgrade from the D3 to the 900BT? No. My out of the box thoughts:
1. 900BT bluetooth sucks compared to the D3. Takes forever to connect, and calls sound like crap on both ends.
2. 900BT maps LOOK nicer, but without modding, they aren't anything different in the way of data.
3. 900BT takes FOREVER to boot up. And unlike the D3, the steering wheel controls don't work until the unit is fully booted. (can't hit mute or volume down for example)
4. 900BT lacks all of the "staging" options the D3 had, and thus doesn't have nearly the sound of the D3.
5. The voice activation stuff is pretty much a gimmick in my opinion. I was born and raised in the US, I have no accent, and I speak proper English. But the 900BT understands me 60% of the time at best. And the "Activation" button can't be assigned through the PAC interface, so you still have to hit the button on the unit just to get it to listen for a command.
On the other hand, the 90BT/900BT is winCE based, so it's moddable. And I always like things I can make changes to. Spend some time browsing through the different things people on the forum have done to get an idea of whats possible.
If I were faced with a choice of the D3 or the 900BT, it would be a toss up. But I certainly wouldn't buy a 900BT if I still had the D3.