C9 everytime. The Falcon will be back a lot sooner and is available from a Level 21 ticket. The C9 is really rare and I think you can only get it from a year ticket.
I don't think this can be confirmed. Maybe the Falcon is more common in your UCD so far, but this won't be the case for anyone. I'd go for the Sauber C9 though - it's obviously a faster car and it is better to show off online.
If you have the cash, always buy all the cars you can. It will save a lot of frustration later, believe me. Still trying to get the McBMW F1 LM car cos I was too tight to buy it the one and only time I ever saw it in the UCD.