I have a an eheim in the larger tank and have no problems at all but have a superfish filter (Even cheaper than fluval) on the smaller, but have had no problems with either but I was to get another I would go with eheim.
I had a Fluval 303 for over 20years running with no problems at all .
I know nothing about eheim.
have fluwho even been around for 20yrs ? i thought 20 years ago people were still using under gravel filters
you just need to look at the way the 2 comanys have put their filters together to see which is the leader in the market
the fluwho fx5 look like a septic tank and the eheim pro3 is a work of art
fluwho put sponges in their filters as they dont make any good media them selfs which tells me that eheim have done far more resurch than fluwho