I use to use Emperor400s.. great filters, good quality and they will keep your aquarium spotless. The biowheels are reliable and work really, really well..
I got tired of the maintanance though. They get noisy over time, the filters are expensive and cleaning can be a pain in the neck. It's not too much.. you know, typical every couple of months.. Turn off the filter, pull the filters from the back (dripping poo water everywhere), replace them with $30 filters, turn the filter on and watch as all the waste that settled poured out of the outtakes into my aquarium. Avoidable only if you took the filter off of the back, which isn't fun with the Emp400. I just got tired of it.
Bought the Eheim 2126, it made a big difference. It is dead silent, extremely easy to maintain, cheap filters, etc. I pull a lever to disconnect the hoses, take it to my bathtub, dump it, change the filters, hook it back up and I'm done. 0 mess (unlike every HOB I've ever used).
Even after 6 months use there is no scrubbing, no pulling stuff out of the impeller, no scraping hard water stains off, no pipe cleaners, etc. Just a rinse with hot water (for the hoses and basket) and I'm good to go.. though I imagine i'll have to do some of that maintanance in the distant future, as of now, it has required about 1/8th the work and money of any of the popular HOB's I've used (aquaclear, penguin, emperor).
best part is that it ages well. I could rinse this thing with a cloth, it woudl take about 15mins and make it look brand new. The spray bar is the only thing that has that aged look to it. The inside of the filter, the powerhead, all of it is in perfect shape.
Forget about the price.. with the cheap filters and easy maintanance this thing will pay for itself within the first couple of months, easily. I can only recommend the Eheim though (one of the more expensive filters). Not had any experience with the other canisters.. but from what I'm hearing, this Eheim is special.