I am putting in a sediment filter in my house, dual gradient density cartridge. I have quotes from both Kinetico and Culligan, with Kinetico being 50% more (several hundred dollars). It appears to me from the literature that both use Pentek filtration/filters. I am pretty sure both are just the "big blue" filter housing.

Is there a reason to spend more ? Are there different types of big blue housing ? (the inlet sizes and dimensions are the same)

Depending on the plumbing SFR you might need to use two Big Blue 4x20 housings in parallel to get sufficient flow rate.

When you install, insert a 1/4 turn ball valve so you can cut flow when changing cartridges and install a pressure gauge before and after the Big Blue housing so you can monitor pressure loss across the filter and will KNOW when to change the cartridges.

I have to figure out now if I want one or two 4x20 housings since the pressure drop across one is 1psi at 20 gpm, which isn't very much of a pressure drop at all.

Is that spec across an empty 4x20 Big Blue?

Should vary depending on what cartridge in installed in the Big Blue and will change as the sediment builds up in the element.