There really is no debate. The AC110 is better in every aspect. The ONLY thing better about the emp400 is the price, and even that isn't much of a difference. If the emp400 was half the price of the AC110 it wouldn't be worth it.
The AC110 offers a much higher flow rating (100gph more that the emp400)
Media capacity comparable to a canister rated for the same sized tank. (just about 1g of media volume, which is more than some canisters. I can fit all the media from my eheim 2227 into an AC110).
A media basket that allows you to customize your media regimen. (unlike the emp 400 which uses those crap media cartridges)
Rinsable mech media (no need to buy expensive media cartridges. Just rinse in a bucket of tank water and put it back in the basket)
Lower electricity usage. (something like 5-10w less. I don't know off hand, but I have the info @ home). While this isn't a big deal for most people, it can add up if you're running multiple tanks.
Reliability. (my oldest AC110 is just about 9 years old. But I have spoke with people who have AC500's twice as old and still kicking. Also, the filter sponge will only need to be replaced every 5 years or so)
... And that's without going into the built in re-filtration system. Or the useless biowheels and spraybat, but that's for our discussion all together.
But in all fairness... The emperor is something like $15 cheaper. Hahaha.