We use both products for canister filters. The Fluval is a 304 and the Ehiem is a 2026 Pro. They are similar in size, but the Ehiem has a higher flow rate.
We like the Ehiem better for several reasons.
The Release mechanism for the tubes is much easier to use on the Ehiem.
The Priming mechanism on the Ehiem is a large (about 4 inches wide) button that you can press down with both hands to prime, whereas the Fluval has a very thin plunger you must repeatedly plung to prime the unit.
The tubing in the Ehiem is normal Green tubing. I believe the green color inhibits algae from growing inside.
The Fluvals tubing is a corrugated opaque plastic. It kinks easily and you must use brackets where you want the tube to bend, or it will kink. Since the corrugations hold air, it is also more difficult to prime.
I give this one to Ehiem, hands down.