I've used the Fluval 304 and it was a royal pain in the neck. Every time we went to change the filter the canister would never set right, the o rings cracked and we constantly had to have them replaced. Many times because of the canister not setting properly the filter would begin to leak out water. We'd have to dismantle the whole thing until we could get it to set properly so that it wouldn't leak. Even the people at the specialized fish store were having problems with it. The final straw was when we had a hurricane come through a few years ago. We had to evacuate and when we came home we had no power. The filter had done this before, but because the power outage was so long (a whole week) it was much wose then before. When the power went out, instead of the filter shutting down, it proceeded to drain about all but 10 gallons out of my 75 gallon fish tank. We had massive water damage from the water that the filter leaked out everywhere. We had this Fluval for three years or so and had nothing but problems with it, from it draining the tank, to leaking, to cracked o rings.....the people at the fish store said that this was really strange as the people they see usually say they love their Fluval and that it works great. Maybe we just got a defective Fluval.... but needless to say the Fluval's since been trashed and we're now using an AquaClear HOB.
My recommendation? See if there's something you like better, or at least if you get a Fluval, stay away from the 304 model and regardless of what Fluval (or any filter for that matter) see if it has a warrenty on it.