Trade 'em both down; Why not?
Because you want blue-chip players?
We could probably be creative enough in trading down to have evey pick in the 7th round. That's 32 picks, man! (Actually, there is probably some rule against that). But the point being that trading down means trading for a lesser player. At some point, you want studs on your team. Gimmie one Terrell Suggs in the first instead of 2 Bradie Jameses in the third.
Trading down wouldn't be bad if we are supremely confident that Parcells can find gems later in the draft. But usually there is a REASON why people are later draft picks. They are more of a reach; less of a sure thing.