Depends on the situation. I took some screenshots but I can't find them on my computer, where does the game store them?

I always use towers regardless because the towers at "Fire on Will" can take down an entire unit to half strength with its ballistas. So when the guys come in, half of of the job is already done for them.

Ladder units make my guys open to enemy fire and I'll lose some, more if there's an archer unit on the wall. Ladders are good if the defenders are hoplite - related because they can't do jack on walls.