I mostly use towers, usually only about 3 soldiers die ( stone wall ).
however ladders are good at getting the whole unit on the wall in stead of like 30 soldiers ( huge unit size. ) which isn't good because when you want to assault the city plaza you need to wait for the soldiers that are still at the bottom of the tower.. which sucks especially when you're low on units and every soldier counts...

sometimes the AI only sends in their first wave of units ( the ones holding the siege equipment ) and then just wait till the timer runs out, with plenty of infantry still waiting to be sent up the wall..
that's how I won a siege versus the Scipii playing as Gaul, I had 3 chosen swordsmen and they had a full stack balanced army..they only sent up 3 units of principes which were easily killed by my swordsmen. and they could have won that time, what a waste...