Hi there kc kong, as you well said it really depends on other factors, for instance, in a 1sq meter tent I wouldn't go 12/12 FS and in a pc box I wouldn't veg for two months. But lets imagine that you mean if there is a difference between 12/12 FS and vegging, I would say yes, there is a difference because if you veg the plant will get bigger and therefore she will yield more
Now, if you are thinking about something specific let us know and maybe we can come up with some more information