Neither one is my favorite. If I am forced to choose between the two evil, I will pick AC because it has a simpler design with less parts to go wrong. Emperor 400 has the biowheel and spray bar that is more trouble than worth it. There are more parts that can go wrong and energy is diverted to drive the biowheel that could have been captured for higher flow. AC is the most popular filter in MEK because many members keep monster fish and AC can deliver the highest flow of all filters. But AC is no perfect angel. It's the only HOB you need to remove the motor to clean the impeller and over time, the O ring will crack and leak. The reusable foam is the attractable feacture of AC but an ineffective mechanical filter because of the large pore spaces that let fine to pass through. I have no interest in cleaning dirty fish diaper and much prefer to dump and replace it. A clogged up foam can pop up, allowing unfiltered water by pass, and potentially drip water From top to the floor.
My favorite HOB is the cheaper and simpler version Penguin 350 which I run without the troublesome biowheel, rendering it straightly as a mechanical filter. The narrower box allows the tank to be positioned closer to the wall. The limitation of all HOBs is the media capacity, not the flow, so why chase for the highest flow AC and Emperor can deliver. You can buy two Penguin for the price of one AC or Emperor for equivalent flow with more total media capacity and better flow distribution. And if you want to save money on filter media, you can buy poly batting from craft shops for penny and cut your own.