Depends on what you're looking for really. Castillo is going to hit more homers, and should theoretically steal more bases, but he's probably more like a .280-.290 hitter. Sanchez is legitimately a .310-.325 hitter, and I think he has the power to hit 15-20 bombs. If I had my choice, I'd take Castillo. That's ironic because I drafted Castillo in my NL-only, then dropped him during his big slump and I now have Sanchez.
Anyway, in real life I think it's Castillo in a walk, because he's one of the top 2 or 3 defensive second basemen in all of baseball.