My thinking is, the less parts, the better.
My friend gave me an emporer 400 when she gave me her 70 gallon tank (yay!). I tried putting the thing together, but I found I was missing a few parts... that turned me off right there. The aquaclears have far less parts... all it has is basically the intake, the motor and the media basket. I've always had aquaclears on my tanks. From the smallest community tank (AC mini), to my monster arowana tank (2 AC500s with a UGF). They've never failed me, and I will continue to use them as long as they are made to the same quality they are made now. Another thing I like about ACs is the fact that they come in so many different sizes. It makes adding them more versitile imho, as they can work on many different sized tanks.
Although biowheels seem to provide pretty good biofiltration, I'm not a fan of them. I cannot stand the sound of splashing water. My aquaclears are almost silent (especially the larger ones). I just raise the water level to the lip of the filter and that virtually eliminates the noise of falling water.
Like I said, I've had ACs all my aquria life. I never had a problem with biofiltration when it came with these filters, whether it was a heavily stocked tank, or a lightly stocked one. IMHO, I don't need biowheels.
-Richer