I rinse and replace my cartridges, too, but eventually they get to the point where they slow the filter too much and need to be replaced. My biowheels are already two years old and very slow, so low flow rates can stop them easily. The cartridges do last quite a few months before this point, though.
BTW, two AC70's would be overkill, but "perfect" for that tank. I like a lot of filtration and two of those filters would give you a lot of media options. Personally, I use a sponge, ceramic, purigen pouch, and ceramic, respectively, in my 70's. They're cheap, too, which is a plus.