To me it's Byrd. I'm never a big fan of putting big money into an offensive guard. I just feel like no matter what level the player plays at, it's easy to find interior offensive linemen who can do the job. Byrd is a playmaker in the secondary, and in my opinion elevated his game last year to become one of the better all around safeties in the league.
I want to keep both, but Byrd is the priority....IMO
With that being said, if you want to keep them both and you want to get an early start on it, I would sign Levitre first. I believe the franchise tag number for a safety would be significantly lower than for a G, since offensive linemen all get franchised at the same number, Levitre would make the average of the top 5 LT's in the NFL even though he is a G. It would be easier to tag Byrd if the situation came up