What are the pro’s and con’s of each? At 18 years old, do I even need these supplements?
The mercury in fish oil is disconcerting, and the phytoestrogen in flaxseed… well… I don’t know what to think. Help a brother out 
Flax contains essential fatty acids - that is, oils only found in plants. AFAIK, the oils in fish oil are omega-3 fats, no necessacarily EFA’s, but still healthy and useful.