Wet dry is more efficient even with freshwater. You can get by with a third the biomedia needed in a submerged system due to oxygen availability. You can also go waaay bigger then anything you can stick in a canister, as in the above example.
But a small tank top wet dry can do the work of a good size canister, and I have run these for years. I only moved to canisters recently to give them a go and try something to reduce evaporation and make the tanks look a bit better... The tank top wet/dry systems I made had big powerheads in the tank I didn't much like the look of... And I got the canisters cheap... And I just like to try new things now and them. The canisters work great, but I the wet dry filters did more with less...