When they're both at their best, Cisak is the better shot stopper, which IMO is the most important quality in a 'keeper. I've seen a few goals go in and thought 'Cisak might have saved that', for example the goal VS MK Dons the other week.
But I think Bouzanis is stronger on crosses and kicking, and I also think that his attitude is better - you've seen when he sprints up the pitch to celebrate. I'd pick him just because he seems to be the more determined and fit into the team more than Cisak, and the difference between them both is only small.