Posted a detailed description of situation before but was likely too long & confusing.
Anyway, Well built, insulated and efficient house in central Texas has a first floor of 2740 sq ft. About 600 sq ft has been very well cooled by an old '86 Trane 1 ton HP and it's kaput. The remaining 2140 sq ft is very well cooled by old '85 Trane 3 ton HP, cooling very well but has issues and it needs to come out too. So 4 tons cools the house very well, we usually keep it set at 74-76. With the small AC out and a fan blowing cool air over there it's still only 78 degress on the small side so this is a good house.
I've decided to replace both units with one 4 ton Infinity HP 25HNA6, of course with matching AH and also Air Purifier. The duct work tie in is no big deal and of course the system will still have an air return on each side.
The question is; is there a real benefit to the additional cost to go with a true 2 zone - the 2 dampers, control board and the second sensor? It's an expensive option. I know people love it, the comfort aspect, but it is truly worth it as a ROI?