Bigger is generally better, especially with my size pool. I upgraded from the 420 to the 520 when I remodeled my pool last year. I would not recommend the 520 for your size pool. The 4 individual cartridges are big and they're heavy when waterlogged, so it's a pain if you have to carry them any distance to hose off. The 420's carts are quite a bit lighter and easier to handle.
My filter uses cartridges, so no backwashing. I have to disassemble the filter, remove the cartridges, hose them off, then reassemble the filter. It's not fun, but the only difficult part is getting the belt clamp back on and uniformly tight enough to prevent drips. That part is easier with 2 people.
