Casspi isn't a great defender, but he has the defensive size and athletic ability that negates some defensive deficiencies.
I don't want another undersized wing situation like the Spurs had when they trotted out Hill, Mason, Bogans off the bench in 2010. Size is very important on the perimeter on the defensive end, as long as its athletic size -- which Casspi has.
And yes, Tucker is the better, good defender.