Hi. The other posters have answered really well, I'll add 3 things.
1. A really big advantage to a wet/dry on a tank with an overflow is that the water level in the tank remains constant. The water level changes in the sump, not in the tank.
2. The second point is more theoretical and less practical. Two things define the performance of any biological filter. These are surface area and available dissolved oxygen. In a wet dry filter, there is a lot of surface area on the scrubbies or bio-balls. These are covered by a thin film of water that exchanges oxygen really well. Under these conditions the bacteria eats Ammonia and Nitrite at an exceptional rate and reproduce prodigiously. In an canister, corner, or HOB fiilter the biomedia is clogged up by organic leftovers. The Wet/dry prevents this because the constant flow conditions and a little help from gravity keep organic materials from building up on the media.
3. The reason these are questionable for a planted tank is because the gas exchange works too well. The gas exchange is too efficient and there is very little CO2 left in the water. This doesn't mean that plants are impossible, they just won't be at their best.
Good Luck!
-ellie
