I would agree you lose fewer with a sap hole run than a siege tower IF there are no enemy missile troops on the walls. Jupiter help you if there are two units of archers on the walls when you make that run though. When you have a siege tower, the archers try to set it on fire first (in my experience) thus keeping the casualties relatively low. Plus, it is much easier to clear missile and skirmisher troops off the walls with a tower assault.

So... Sapping on undefended walls, towers on defended walls. Ladders for the cannon fodder.