The 32MM or 1.05 will make no difference in flow rate on a 1.5 cu. ft. system, but... use the 32mm. The 32mm is not used by some assemblers because it saves them $2 in assembly cost. When you sell a thousdand units a year, that adds up. There is no disadvantage to using a 32MM other than the cost. My personal recommendation would be to go with a 2.5 Cu. Ft. system, this will give you great flow rate potential should you actually use multiple bathrooms at the same time, and set the system to a very efficient salt setting, 4 pounds per cu. ft. The 2.5 cu. ft. system is the largest system before the system size gets overwhelming, and the costs are still reasonable and the minimum recommeded flow rates are still ok. The 7000 is also available with 1-1/2" connectors, so the appearance of dropping pipe size can be minimized. Code does not allow a softener to drop pipe size, but it is common practice by many companies. Even the 2 cu. ft. system will be fine, I would recommend against the 1.5, it is simply too small for a larger house with multiple bathrooms from a technical standpoint. I know many peoiple install the sears unit which has a peak of 8 GPM in their mega houses and they are finwe with it. That does not make it correct.
Let me know if you need a referral to a company that can do it correctly, I work with over a hundred companies local to you.