Gosselin has Smith as his 7th rated player. I think Dallas has him rated higher on their board if they have him rated higher than Okung (#4 on their board last year). Also, some teams would rate Smith lower due to his inexperience at LT which shouldn't be an issue with Dallas. IMO, the Cowboys have him at around #5 on their board.
Costanzo is rated #16 on the Gosselein board and I think there will be some correlation with the Cowboys rating of him. Another frame of reference is Brian Bulaga, who I think is a comparable player to Costanzo. The Cowboys rated Bulaga #13 on their board last year. Dallas may like Costanzo a little better than Bulaga (as he might have better LT potential), so I think Dallas has him rated around 11 on their board.
So if the choice is between Smith and Castanzo at 9, they should take Smith. And Dallas shouldn't even entertain any trade down where they could lose Smith and pickup Costanzo unless they can get at least a mid-2nd rounder in return.