It's more about what each can offer you. Realistically, how many years does Moss have left at an "elite" level? One or two maybe? Marshall is going to be 26 in 2010. He has at least 5-7 "elite" years left in him.
Moss is still a very good WR, but at this point I would much rather have Marshall. As Moss ages, he is losing the most important aspect of his game: his speed. His best asset is his ability to get behind defenses and stretch the field. Marshall doesn't have that type of speed but he works the middle of the field better than Moss.
We would be crazy not to trade Moss for Marshall straight up. But it will never happen. Marshall is worth more than that.
You bring up the numbers, but also take into account Marshall produced that with Kyle Orton as his QB. A banged up TB is still leaps and bounds better than a healthy Orton. Moss was banged up too, but at this point in their respective careers it would be crazy to say you'd rather have Moss than Marshall.