Doesn't matter, both are big needs. What the Cowboys need to do is sort of a Mike Ditka type draft, except giving up all the draft picks for one player, do it for 3 players, or 2 if that's the best he can do. This team doesn't need more average players, they have enough of those.
DT, OG and S should be the 3 picks.
With only 7 rounds of the draft there are plenty of JAGs they can sign cheap for the players they lose to free agency that they don't want to or can't keep.
Of course, you've got to keep the receiving corps and the DE's intact, can't rebuild the team in one year....