Nikon 28-200 vs. Tamron 28-75. Help!!!

[kimmeraz]

So, my awesome Grandpa is buying me a Nikon D700, which is a full frame camera.
I've opted to not get the kit, due to the fact that the lens that comes in said kit has been rated by some as one of Nikon's worst lenses ever made, so why waste the money right?
I currently own only ONE lens that will work on this new full frame camera, and it's Nikon's 28-200 3.5 that was discontinued back in 06'. (The rest of my lenses are DX, which really won't do the trick with the new FX camera.)
But since I'm not getting the kit lens, I do have the option of adding on something similar in price which would be the Tamron 28-75 2.8.
Now my question is this: Would it be beneficial to have this lens since the one I already own covers the same focal range? Yes, the Tamron is faster, but is that really the only benefit? I'm really struggling with this decision, so any input is appreciated!
Oh, and if you have any other lens suggestions, please list them! Thanks!!!