Emperor 400 is perhaps the top of the line in Emperors. It pumps 400 gph. THere are two baskets that hold biomedia - mine are packed with ceramic cylinadars but you can use sintered glass, lava rock, small bioballs or other creative things. There are two cartridges that do the mechanical filtration. Two biowheels finish it off.
The biowheels are driven by spray bars, so they do not rely on speed of filtration. The cartridges can be rinsed and shaken at each change. I think mine only need replacing about once every 6 months. Because there are two of them, you can do one at a time and never lose any cycle.
My 400 does not selfprime with out a bit of grinding and work - something I am loath to let it do. In the times of power outages, though, it has self primed. The three I have have never failed me in any way.
They can be found relatively cheaply - particularly on line or during a sale.
The 400 is rather wide - it takes up the majority of the back of at 20-30 gallon tank.
I really like the Emperors. They are my favorite of all filters.