Chances are, given the contract situations and cap hits, as well as other factors (such as depth on the roster and supporting talent), nobody would make the trade (the Vikings wouldn't want a QB looking for a huge payday after such a short body of work and the Cowboys already have two decent [but not great] RBs, whereas a third would cause a lot of problems and confuse an already good rotation). However, if you put up Romo and AD in the FA block in 08, said both of them want the exact same terms on their contracts, I think Dallas would go after Peterson first (Julius is leaving and that would set the position for a decade or so and Jerry could pick up a top flight QB by trading up or depending where Cleveland falls, that pick) and the Vikings would go after Romo (Jackson isn't going to cut it and Romo is provides more stability and they already have Chester Taylor, who is a good RB in his own right).
Now if the question is asked in a vacuum (ie, both are FAs and you are starting an expansion franchise), I think Peterson would be the smarter pick. Romo wants elite money (and for a QB, that's a lot more) and is a bit older. You would get your choice of QBs in the draft. But that's just me.