What kind of tanks are you gonna be comparing? Planted vs reef? fish only vs fish only? There is not just one FW tank, just like there is not just one SW tank.
Are you trying to show how one is more expensive than the other? The argument could be made that both fish could be kept in a tank with an UGF. The only difference in price would be the salt(which isn't really expensive). If it's a comparison of a high end planted tank VS a reef tank. The price would probably be identical. If it's the cost of the fish for each tank that depends on the fish you choose. You can buy about 6 clown fish for the price of one discus.
Is your paper gonna be about maintenance? Again both sides have advantages and disadvantages. A discus tank you should change water on an almost daily basis. A fish only SW tank you would have to top it off with freshwater daily.The planted and reef tanks need a lot more time and attention given to them. Both would need checking of trace elements. The reef tank would have a few more tests, but nothing to give one the edge over the other.
I think it all comes down to personal preference, You can find pros and cons for either. I'm not sure if you picked a very easy topic for your paper.