I think I might be of help here. I have had two "classic Eheim's", and I currently own a Rena XP1 and XP3. I found nothing from a quality standpoint between the canister/ motor assembies that makes the Ehiem superior. In fact, the Eheim canister slipped out of my fingers from less than 6" above the sink and it cracked, thats why I say "had" an Eheim, and sold the other which I never really used. Also, a friend of mine was looking at a used Eheim in a LFS and he broke a latch on it, I don't know how either, he is a big Eheim guy. From a features and layout perspective the Filstars are more comparable to the Eheim Pro than the classic anyways. The Filstar does have adjustable output, but I don't use it. But here is my long take on the comparison:
Things I liked about my Eheim:
1. Quiet
2. One piece inputs/outputs
3. You always know you will be able to find parts for it, Eheim reputation.
4. Media, their media seems to be superior to others I have seen, but it doesn't come with the canister, so its kind of a mute point.
What I didn't like was the following:
1. No integrated shutoff on the tubing, makes cleaning a lot more of an issue. Yes, priming is an issue because you almost have to empty the tubing to clean the filter.
2. All the media is just dumped loose in the canister. So, if you want to clean the media or change something, it is a chore.
3. The tube that comes out of the bottom of the canister makes it a little awkward in tight spaces, I know, the Rena's have a larger footprint to start off with.
4. Proprietary tubing that almost requires you buy Eheim accessories($$)
Filstars:
Likes:
1. Integrated shutoff/disconnect, I unplug, flip the lever and grab the canister, run water over the media in the baskets, put it back, flip the lever and it refills, plug it in and done.
2. Filter Media Baskets: Media is in seperate media baskets that lift out of the canister easily. This makes cleaning, media changing, etc very easy. Also, you can use any brand media, I do.
3. Quiet as Eheim IMO.
4. Easy impeller maintenance
5. Standard size tubing, I use generic surface skimmers with mine and they work great and fit.
6. Many different input/output options come standard saving cost at startup, but see below.
Dislikes:
1. Input/output options are just press fit togther in many peices. My filters becoming fountains are a big worry for me. I ended up siliconing the first two pieces that go over the side of the tank to make sure. Never actually had a problem, but just to be safe.
2. plastic on in/outs fade, my XP1 is over three years old and the top side of the spary bar is a little faded, probably from the 350 watts MH that was baring down it
Obviously, my vote is for the Filstar XP2 or XP3. I know Eheims have a great reputation and you can't really go wrong with one. But, the added features and ease of maintenance at less overall cost make all the difference to me. Also, it seems like a lot of the people who prefer the Eheims, have never owned a Filstar. For years, I could see that it was the only option.
Disclaimer: None of this is to be taken as hard facts or insults, it is just my humble opinion from my experience.