I am planning on ordering a fleck 5600sxt valve softener and am confused about what size to get.

I am on city water with hardness of 18gpg, live alone in 2.5 bath house and am guessing I use about 75 gals a day. If I understand right, 1 person x 75 gallons x 18 gpg x 7 days = needed regeneration of approx. 9500 grains once week. Is it more efficient for me to get a 32k (1 cu ft resin) or 24k ( 3/4 cu ft resin) setup?

I know these questions come up a lot but could truly use some advice. Thanks, Noah