Funny that you should ask. I am currently running, on my 90, a fluval 304, aquaclear 300, and an emporer 280.
In my opinion that ac and the emporer are pretty close. I find that the emporer does not prime as well as the ac. If the water level gets too low the emporer looses suction before the ac does. Although this is only during water changes of over 30%. The water level has to be down at least 4 inches before I notice a problem. Noise level is a trade off. The ac seems to have a quieter motor but also has more water noise if you levels are not right to the top.
In my opinion I like the ac better for mechanical filtration. If you are getting a fluval 304 then the biological side should not be a problem. To me the only added benefit of the emporer is the biowheel. It all depends on what the main purpose of the filter is going to be. From the sounds of it you are getting the HOB for mechanical filtration and in my opinion I would go with the ac.