With Filters, bigger is better. The rule of thumb is when your clean pressure gauge reading increase 7 - 10 psi, it's time to backwash.
The Pentair 2000 vs 4000 series are the same filter but with either a stainless steel tank (2000 NS series) or a fiberglass tank (4000 series - hence the F in FNS).
The frequency of backwashing depends on the pool usage and the amount of debris entering the pool.
I have a cartridge filter that I clean about every other month. I have a 19K pool, not a typical screened enclosure around the pool as in most South Florida pools, so I get my plenty of tree debris in the pool. My suction side "Great White" cleaners also has an inline leaf trap (the best thing since sliced bread and salt systems), that traps a whole lot of debris before getting the pump basket and cartridge filter clogged up. I simply clean that out when it gets visibly overloaded with "stuff".
Unfortunately, the little floaties in the water, especially at night with the pool light on, will occur with any of the three filters, although the DE may be capable of removing more than a cartridge or sand filter. You will still see that at night.