The 900 will have a metal tank as well...I believe only the pre-S2R 800s have metal tanks. Metal tank is the way to go IMO so you don't have to worry about swelling. Get one with adjustable suspension if you can.
I used to have an '01 M900. It was a great bike, and my favorite Ducati motor to date.
![waytogo](http://ducatimonsterforum.org/Smileys/classic/waytogo.gif)
I believe the metal tank is also a little bigger (holding more fuel) which is nice for longer trips.