I think FFXIII is almost certainly going to sell less than FFXII.
1. It's on a system with a much lower install base (not terribly important since I think the fanbase will follow the franchise, but still bears mentioning).
2. The main series has been on a decline since FFVIII.
3. The Final Fantasy brand name has taken a slight sales beating from being heavily milked, even since FFXII released.
On the other hand, it is the return of Nomura/Kitase to the main series, so that might bring some people back... I guess we'll see, but my gut and at least 3/4ths of my brain says expect something lower than FFXII.