That magnum 350 is a great filter and has been around for about 20 years. They are powerfull and quiet.
The point of the canister filters is that they have more powerfull motors and much more room for media. In the same lightly stocked tank you can run a large canister like the fx5 or mag 350 for several months before you need to clean it and rinse the media. There is no cost involved in that since the media can be rinsed many times and only needs to be replaced when it starts to fall apart. In an Emporer or other HOB that uses the flimsy filter pads you need to clean them once a month in the same lightly stocked tank, maybe more, and when the filter pads gets clogged all the water goes out the bypass and is not filtered at all. That does not happen in a canister, they don't bypass.
In the end it is all personal preference. The costs are pretty much the same. Large canister equals better filtration and less maintenance with just one inlet and return. You also can run a heater inline and inject co2 in the return line as well as UV. HOB filters can't hide your heater and you will have several inlets and returns to do the same amount of filtration plus cleaning more filters more often.
And yes, a sump can be even better at all those things but not nearly as easy to setup.
If you are comfortable with your HOB filters there is nothing wrong with them but there is a reason they only make them so big. Once you get to larger tanks they are not a match simply because you would need too many of them.