The difference in sponge porosity can't be argued, but the sad fact is, the finer the pores are, the faster they clog up with gunk. I have some fine pore sponge prefilters that may clog badly in days, where a coarser one will take far longer to clog, and the super coarse pond type I use now rarely, if ever, clog.
I don't think using a coarser sponge necessarily means you get a downgrade in filtration, since I think of sponge primarily as bio media for growing BB, rather than mechanical filtration, though sponges certainly do absorb plenty of dirt.. I prefer floss for mechanical filtering.
And I agree, those refills for filters, made by the filter manufacturers, are a cash grab for them. Many have carbon in them, which you might or might not be aware of, but none of them needs to be replaced the way instructions tell you to.. in fact, doing it the way they advise means you toss away vast amounts of the precious BB you rely on to convert ammonia every time you make the change.
And carbon has a big disadvantage I think too many are not aware of as well. If you leave activated charcoal, which is what we refer to as carbon, in a filter for too long, it will simply discharge all the organics it has adsorbed back into the water.
So it is far better not to have it, than to have it and not change it often enough. And how often it needs changing can be difficult to judge, as it depends how great the load of organics the carbon has to adsorb. Light load, it will last longer, heavy load, might need replacing weekly.
Carbon is best reserved to use when you must remove medication, foul odours or perhaps unwanted colour from the tank water. Wood often turns water yellow or brown, carbon would help reduce that, for example.
I keep thinking I should try to organize a group buy on filter floss. I can get an entire bolt of the stuff at Fabric Land for 40% off the going price per metre any time I want, I have asked them about it.
All I have to do is compare what that price would be, to the price of the largest roll Big Al's sells, whether it's per sq. ft or sq.m, to find out if it is worth trying to do it. I believe the bolt material is a bit thicker, but again, I'd have to do a close comparison to know for sure.
But it's for sure the cheapest and best water polisher for the money, and it can grow quite useful bacterial colonies too. It doesn't have to be tossed every filter cleaning, it can last a surprisingly length of time.
If I did buy a bolt of floss, I'd need a ton of folks to share it, cause the bolt is nearly three feet in diameter and 60 inches wide, I think they are 25 or 30 metres long. I have no place to keep something so large.. just getting one into the back of the van would be a feat.