Eheim / Fluval
Coke / Pepsi
Nikon / Canon
Hatfield / McCoy
I have both.
I have 8 tanks - so I just sold my Fluvals so I can swap parts and filters back and forth as I clean and as I move stuff.
If you were to pile all the parts of a Fluval *06 onto the floor, and then pile all the parts of an Eheim classic next to it - the Fluval pile would be about 4 times as high. Hoses with elbows to get them over the tank, a big double tap, baskets, some kind of pre-frame thing, foam, plungers, etc, etc.. .its endless.
Fluval makes a fine filter. My 204 was the quietest filter I've ever had or seen or owned... People talk about filters being dead quiet - this one was. My Fluval 305 I sold 4 days after I bought it... it was way, way too noisy.
That said, Fluval and Eheim (classic) couldn't be further apart in their thought process or how they're designed or made. Classic: in at the bottom, out at the top. Holds TONS of bio media, is simple, is quiet, normal hoses, brilliant double tap design makes for bone-dry moves, rescaping and cleaning. Easy to prime and start... start the siphon, fill the can, plug it in. I don't get these people that find an Eheim classic tough to start. Easy to clean. Pre-filter in the tank, pop the top, rinse the sponges in aquarium water, re-stack. Takes 5 minutes. I don't get these people that find a Classic hard to clean.
I've become an Eheim guy. I have 10 of them now. I've tried several others, and I just love them. The classic is preferred for me, but I do have a few Eccos.