Both are brutal, disgusting tyrants. However, Pinochet was defeated for re-election and left. The Castro regime has suppressed democracy for decades. So Castro is clearly worse.
Pinochet did not want to leave. He was forced out after others decided that falsifying the election results wouldn't end well for anyone related to the regime.
Of course he didn't want to leave. No one ever wants to leave. Fact remains that he didn't tighten his grip on the country's neck enough to stop it from happening.
Pinochet may have been the more brutal of the two - Castro seems more fond of disappearing people into prison for life than public executions - but there's still no question that when you compare Castro's 50 years to Pinochet's 12, more people have suffered under Castro.