Yes, more crits is better I'd say, because of the effects. Also you are spreading your chances of getting the really good ones such as 4-6 and 6-6.
Actually there is one factor I forgot in my stats, and that is bulkhead hits. TD and DD both do 1 point of damage for bulkheads, so that would make 6AD DD slightly better than 4AD TD.
And if your target has Adaptive Armour, DD is better since you'll never lose out on any damage due to rounding down. 1 hit DD does the same damage as 1 hit TD against Adaptive Armour.