generaly speaking you are more likely to suffer lower losses if you ai a seige when you are attacking (not if you are the defending party). this is becasue the autoresolve doesnt seem to take into accout the walls etc very well and calculates as if it was a field battle - if you have more soldiers you will normally win. there are other issues like for example normally when you are layingh seige it is necessary to kill virtually all the enemy troops as they will retreat to the town square and stop routing. the autoresolve seems to calcualte that the army will rout - so if you look at the losses on each side they are quite low, but when you take a settlement all the enemy troops automatically die.

in field battles however there should not be any advantage to autoresolving. it often results in undesirable outcomes such as losing significant numbers of your top quality soldiers, as oten you just seem to lose a percentage of all your soldiers, when if fought out you wqould ideally not lose too much nights for example.