It depends not only on the unit, but on the enemy units, too.
survival bonus from dodge would be:
10 / (enemy accuracy - your unit dodge(wihout bonus))
survival bonus from defence would be:
4 / (enemy damage + your unid defence + 4)
So at the start of the game defence is probably much better. At the end dodge will be probably better, unless AI has developed enourmous accuracy via technology.