Aug 2, 2016, 15:3108/02/16
there are many factors
Pirates takes less resources + Less time to train and have low offense values.
Merc takes more resources + more time to build and have higher offense values
I have different research levels but if i take resources into consideration
Bombraider takes 2166 resources and have an offense power of 357.6
but with 2166 resources you can train 4.33 bucc and that will have a power of 387
so more units of pirates or some units of merc.