First, Rolf C. Hagen Group is a mini pet conglomerate which owns Fluval. You can see all their brands here
http://usa.hagen.com/ourbrands
Next, Eheim is still an independant company, not part of a conglomerate. They pretty much do only aquarium related products.
https://www.eheim.com/en_GB/home
Rena is owned by Mars which has as one of its divisions, pet care. They make food and other products unrelatde to fish keeping.
https://www.mars.com/made-by-mars
If I had a dime for every time I saw somebody claim that Eheim is not what it used to be, I would be very rich. Fluval used to have a rep as crap and I have not yet seen a post about a Fluval or sunsun running great after a decade+.
I use Hydor inlines. I bought my first one to go with my first Eheim and that heater still works great. They are likely the most reliable heater I have owened. I say that also believing there is basically no such thing as a reliable brand of heater. I have had almost all of the fail sooner or later. I can say I have a couple of 18+ year old Ebo Jaegers still running OK. O also have Ebos that crapped out after a few years.
One last note here. It is as much about the filter media as it isa bout the filter. As far as i am concerned the best filtration available are the Hamburg Mattenfilters. I make mine using Poret foam from Swiss tropicals. I also loaded my 3rd Eheim 2026 with only 20 PPI foam in both baskets. There are not floss pads or carbon or anything else, just poret foam. This filter runs on a 75 gal planted tank along with an AquaClear 300. The AC is loaded with two sponges and a layer of floss, Both filters have pre-filter sponges on the intake.
I do weekly 50% water changes and I swap out the floss from the AC and I rinse both sponges in tank water in a bucket. When I set up the Eheim with the Poret, I asked Dr. Tanner of Swiss Tropicals when I should clean it. My other two canisters I have cleaned twice a year since they went into service. He told me to clean it when I can see the output flow slowing. I have now been waiting for about three years for this to happen. I can also tell you that of the four mattenfilters I run, I have only removed one for cleaning after I had it up for 3+ years. I can also guarantee you that my tank water where I use the mattens and the Poret foam cubes likely has clearer water than 98% of the tanks you will see. The reason is simple. Most solid wastes in a tank are organic. In the proper media, this will be broken down and consumed by small to micro organisms.
If you would like to learn the science behind filtration, read these two articles in order:
http://www.swisstropicals.com/library/aquarium-biofiltration/
http://www.swisstropicals.com/library/mattenfilter/