I've owned Emperor 400's for roughly 10 years, and liked them very much.
The bio-wheel works, and is proven to work in independent lab tests.
If you want additional biofilter capacity, you can fill the media containers with ceramic rings, or whatever you want.
There are less expensive alternatives to the Marineland cartridges, including the Imagine cartridge, which has a reuseable frame which takes replaceable floss bags, similar to a Whisper. The beauty of this is that the reuseable frame can also accept homemade pads cut from a bulk roll of floss.
I did this for years, and could get roughly 10 pads from a $5 roll of floss, making the cost of operating the filter something like $1/month.
Mine were stone-axe reliable, the only thing I ever did to them was replace the impellers, roughly every 4 years. They never jammed, or stopped pumping water, but they would get noisey and rattley.
I never had one lose prime and fail to restart after a power outage.
The Emperor is made of a vastly tougher plastic than the Aqua Clear, and can withstand much more abuse. Aqua Clears parts frequently break if dropped, but I've never seen or heard of an Emperor component breaking.
Emperors appear to be made of a dense, slightly flexible plastic which resists breaking, some form of ABS perhaps?
The plastic used in Aqua Clear seems like the styrene used in model airplanes, very stiff and brittle.
I admire the basic design of the Aqua Clear, but the execution in what may be the cheapest plastic available completely spoils it for me.
I don't use HOB filters any more, but if I was going to, and if I wasn't going to use a Marineland, I'd buy a Whisper before an Aqua Clear.