In my book you are walking a line between the capacity of the unit and the flow rate of the unit needed ..
The 1 cubic with a 6lbs setting would do 20k or with the 19grain and 2 people 900gallons on the setting with flow rate of 6gpm and peak of 9gpm.
The 1.5 cubic with 9lbs setting would do 30k or with the 19grain and 2 people 1400gallons on the setting with a flow rate of 9gpm and peak of 11gpm..
Either will work for what you have now, but if there are going to be more people or sell the place to a large family the 1.5 would be better.
Just my thoughts on the matter.