According to our ante-natal class instructor, ventouse is used when they can see the head so they can put the suction cup on, forcep may be used when baby is still a bit higher up. She asked us to write out the possible risks with each method, and we wrote brain damage, to which she said she couldn't find evidence. I would still be pretty reluctant to have them used on my baby.
After reading the link I can only assume my ventouse was because I was so tired, I'd been in labour for 32 hours with no proper sleep! My boy was 7lb 14oz and yes had a bit of an odd shaped head for a couple of days, but nothing really noticeable!! He's now almost 15 and has no lasting effects from it at all!! Xx
From what I've seen on birthing programmes too, I think they take you to theatre for forceps but ventouse can be done in delivery suite xx not sure how true that is tho!! Xx