Is it more important for the Giants to draft an offensive tackle or guard this year?
The specific position is much less important than the quality of the player. Any guard or tackle they do select will add versatility to the offensive line. The Giants could use depth at either spot, so it's best to focus on getting the best available player.
The beauty of the Giants current offensive line is that they can pick the best player and then figure out a way to fit him into the offensive line that most helps the team. Geoff Schwartz has played both guard and tackle in the NFL, and many believe Justin Pugh could play at guard as well. That means they can pick the best player, whether guard or tackle, and plug him in wherever Tom Coughlin thinks is best.
Versatility is the name of the game for offensive linemen. They pride themselves on it, especially in the Giants organization with head coach Tom Coughlin and position coach Pat Flaherty. That's why you see players take practice reps at each position on the line. So I don't think you need to target one or the other because the Giants could use as much depth as they can get on the line, whether it's outside or inside.