I don't think that really makes sense, the consoles are so different that either way it is difficult to port from one to the other I believe.
If you build the game for the 360 first, port it to the PS3, it's probably not going to be optimised for the PS3. Not using all the cores to their full advantage etc. If you build it for the PS3 first... then for porting it to the 360 I would assume it would almost be a case of "downgrading" it as the architecture is more basic than the PS3.