Moved to Game discussion. Please do not use Game Tutorials thread for asking questions. it's for sharing knowledge and guides. 

Ignoring resource cost, which is important but if lets say your win pirates and mercs from the lighthouse. Mercs would be better to keep. They offer more bang for the rum consumption. For instance I marine uses 4 rum per hour while a bombardier uses the same but the bomb has a higher stat. The basic rule of thumb you can assume is the higher the class the better the unit is compared to the prior class.  Also the farther down the list generally the unit is better than the previous except in the pirate class.  Pages 2-4 in the pirate class stats plus rum consumption the first 4 are better in comparision.