The way I believe it's going as far as Flores, there is not a big enough gap between Fitz and Rosen, though Rosen (By camp reports, and the 1st Preseason game) seems to be a]slightly ahead.
Rosen is still not perfect to be thrown out there, and has to clean some things 1st, add to that the O-line is still trying to gel together (They are not going to succeed on talent...at least not for now), so throwing out there the Rookie more then the Vet would make no sense.
Because they are close, Fitz gets the benefit of the doubt, considering his experience, and his knack to pull out 2-4 impressive games before he comes down to earth...So either Rosen proves himself again this week to be much better then Fitz, or game Three (Dress Rehearsal) will have Fitz as the starter for the 3rd straight Preseason game, which also probably means game 1 of the season sees Fitz starting unless in week three Rosen overtakes Fitz.
I have not yet heard or seen anywhere who will be starting Preseason game two, but considering how short Fitz got to start, and Rosen's ugly INT, I believe Fitz will start, but the game Rosen had (Though Coach Flores will never admit it), was not lost on him, INTs happen to best of QBs, and Rosen bounced back amazingly, so though Fitz will start, both will be competing against the Bucs to start Dress Rehearsal, and probably the season...Expect Fitz to get at least 3 to 5 drives, and Rosen to go deep in the 4th quarter after that.