The Lancaster's main escape hatch was in the nose - the bomb aimer was lying on it. I believe that the parachute exit for the SD Halifax was in the fuselage, through the hole that had originally been used for the ventral turret. That would give more space for the agent and his/her kit whilst en route. As someone who grew up in 4 Group territory, I'm reluctant to suggest that the Halifax was given this role because Lancasters performed better in the bombing role, but I can't tihnk of any particular advantage it had for agent dropping.