Besic will help a lot more defensivly than Gibson. Plus he's quicker and this is a very quick fit team. Gibson won't cover half the ground Besic will.
My issue with Besic defensively is that he's got almost zero positional discipline, unless it involves a man marking type job where running round is more the skill required.
He's very prone to following the ball/player to needless positions on the pitch which can leave us more exposed than normal.
But if we're going to play our usual exposed game then Besic's greater pace might be more useful.