The problem with the emperor series is yes it's true some bacteria stays in the bio wheel but you still lose bacteria when you change the media
Also no matter what I always filter at Least double what I need say I have a 40g I buy something rated for a 70g and here is why all filters have a gph
they say or claim meaning gallons per hour. This are commonly over stead but even if they were accurate they get slower with time so if you start higher than you need you will run to fewer problems .
Now back to the canister vs a behind the back i don't service my canister but every 3 months or so and even than I don't throw away the bio balls or anything else I just rinse it in tank water throw away the water in the filter and I am good for at least another 3 months so while it seems they are more expensive in the long run they are cheaper and don't make hardly any noise