Well, I never used Eheim before. I'm assuming your going to put one on each side.. Looks like it has a carbon filter pad, instead of the actual carbon..
I don't know. I like mine because you can change the media to your specifications. I would assume the fluval fx series might be similar..
In my opinion it might be a close call with your 180. In the reviews I read most people have one on a 75 gallon..