I haven't used the Aquaclears, but I have used the Emperor 400 for years. The old models, with the pull tab on the middle back were awesome - I never had issues with them and ran them for years until I was stupid and got of the hobby and sold them. Those things would even run with a chipped impeller and still not make any noise. Now I don't know if I have just gotten a few bad batches or what but the new design on the Emp 400 does not have the pull tab anymore and the lid doesn't have the hole for it either (easy way to ID) but these new ones have been noisier and chatter, I have worked with Marineland before on it thinking it was a bad batch, but even the new ones sent out as replacements were noisy. If I was going to buy a filter today, I would have the person hook it up and run it to make sure you are getting what you want and it's ok. Even then, you take a chance. I just bought a used Rena XP4 and had it tested before taking it home. Upon getting it home I took it all apart to clean it out and found the impeller was melted. I had tested it, but I didn't take it apart since it did push water - $40 reminder.
edit - the Emp 400 also have optional media basket/slides, get them if you can or you can order them too.
