I was influenced by the 5 bathrooms and 3600 sqft.
With your chlorinated water, you will be able to go longer between regenerations than if you were dealing with iron. So the 2 cuft capacity will not go to waste. Yet the 1.5 cuft unit would do nicely too for your regular use. I think the 2 cubic ft unit will be a little more efficient. Bring in the event where you are using the 5 bathrooms, and that extra SFR could be useful too.
Note that you might be able to use even less than 6 pounds of salt per cubic ft. If you are from one of the places that is short of water, Ditttohead may have some tips on optimizing for your conditions. Note that a "48000" unit should not be used to provide 48000 grains of softening. Think of that as a nominal number that really means 1.5 cubic ft, and that "64000" means 2 cubic ft of resin which should provide 30400 at 4 pounds of salt per cubic ft (8 pounds each regen).
If you want to use a cartridge type filter, I like the Pentek Big Blue 20 x 4.5 housings rather than smaller housings. There are a lot of different cartridges available. The Pentek GAC20-BB is a granulated charcoal filter element for that housing that is rated at about 15 GPM.
http://www.discountfilterstore.com/...ivated-carbon/size-advertised/20-x-4-1-2.html is one source. A backwashing carbon filter is more expensive, but if you are also trying to remove chlorine or other chemicals, it might be called for. It would look like a second water softener and cost almost as much. The media in that would last about 3 years or so. Softener resin is good for maybe 10 years, although some get significantly longer life.