Har1, see the thread I started comparing these two lenses.
I am assuming that this is not a money decision since the sender realizes the mistake and you will pay the appropriate price for either lens.
The answer as to which lens is best for you is entirely dependent on your type of photography. I concluded, comparing the lenses, that in most light situations the images are indistinguishable. Only in low light with moving subjects does the 2.8 provide a real advantage. Others may disagree, but that is my conclusion from testing them side-by-side. For most situations I would prefer to have the lighter weight. I will carry it more often and use it more often.