I can tell you why I prefer canister filters:
1. They keep all of the filtration outside of the aquarium.
2. They move a lot of water.
3. They are generally very customizable as far as media.
4. They are perfect for adding accessories like inline heaters and CO2 reactors due to the fact that they use hoses for input and output.
In a planted tank, moving a lot of water is what is most important to me. Mechanical filtraton is next.
Biological filtration is great too, but less mandatory in a heavily planted tank.
Biological filtration does not remove nutrients from the water. It just converts, via biological processes, nitrogen compounds into less energetic compounds. Plants can use any nitrogen found along this whole chain. Granted, ammonia is easier to build proteins from than nitrate, but it is also far more toxic to any fauna you might have living in the tank.