There is no direct conversion between dilithium and fleet marks. about the only thing you can do with fleetmarks is help progress the fleet projects.
However you get fleet credits in return which let you buy fleet gear and items.
The best fleet gear requires fleet credits and dilithium.
So you want both actually.
if you go by the return rate of fleet marks to fleet credits and dilithium to fleet credits you get a ratio of 50:1.