For mechanical filtration I'm a big fan of the AC filters. I've both had AC300 (they've a new designation now... AC70? Not sure, it's the 300gph one though) and Emperor 400 filters. Imho the way the AC is built allows better mechanical filtration, as sometimes I saw water flowing *over* the media cartridge in my Emperor, whereas the AC design pretty much forces the water to flow through the media, plus the AC design allows for more flexible media deployment.
On the other hand, though, the biowheels give massive bio filtration capacity! It all boils down to what sort of filtration you're looking for. Adding additional bio filtration once you have "enough" doesn't accomplish anything other than redundancy (in case you accidently drop your biowheel in chlorinated tapwater, etc
). I would tend to lean towards the AC myself (and, in fact, I ran 2x AC300 and one Emp 400 on my 5'x1'x2' 75 gallon tank way back when).
In the end it really doesn't much matter, as I'm suspect either combination will give you a sufficiency of filtration.
-Rick (the armchair aquarist)