The Aquaclear is much more quiet than the Emperor IME, but you can keep the Emperor pretty quiet if you keep your water level high in your tank. I like to use the Aquaclear with 2 sponge inserts stacked together instead of the carbon. Then I put the Biomax (ceramic rings) on top of that and it's good to go.
Basically, each of these filters has its advantages IMO
The Emperor has the Biowheel, which allows a big surface area for beneficial bacteria to grow. It also comes with an extra media basket (2 for the 400 model) that I use for filter floss. But, the cartridges are expensive to replace, and the filter itself is more expensive.
The Aquaclear lets you use 2 foam blocks, which IME gives you plenty of surface area for your bacteria. Plus, the Biomax is included, and also serves this purpose. You don't have to buy replacement cartridges, since it doesn't use any. The foam blocks can be rinsed out (in tank water ONLY) over and over. I have some that have never been replaced, and hve been running for a couple of years. The Biomax never needs to be replaced at all from what I can tell. But, there's no Biowheel.
I think both of them are good products, but if noise is your main consideration, then go with the Aquaclear. That said, I use an Emperor 400 on my bedroom tank (30 gal
long) and I sleep fine.