From what I've gathered, Eheims are a pain to clean, but usually run w/out problems for years. The actual flow rate tends to be about half of the volume/hour they claim, so don't go with one rated just big enough for the tank. For the money, Fluvals are a better deal, and easier to maintain, but the quality of the materials is less. Sort of like a Mercedes and a Chevy. The Mercedes costs more, but runs longer with less trouble. The Chevy is cheaper, needs more parts replaced over time, but they're easily available.
You can build a canister filter much cheaper than either brand, using a 5 gallon bucket, a powerhead, and a few things from the hardware store. I've seen several DIY projects on various forums, and plan to do something similar if/when I set up multiple tanks with a central filtration system.