Considering replacing 12 y/o 3.5 ton Rheem heat pump which is costly to run and can't maintain my desired 74/72 degree day/night comfort levels when it's 95 degrees outside during the day and only cools down to 78 degrees at night here in central Florida. The outside unit is badly corroded due to poor installation. Previous owner had it put half on, half off the existing cement slab and 2 inches away from an irrigation sprinkler.

Contractor says load calc puts my 1827 sf lakefront home at high end of 3.5 ton single stage or 4 ton dual stage. I have 6 sets of 8 ft wide sliding glass doors to let the outside in on the NE and SE sides, as well as a 350sf family room with an open beam 14 ft high ceiling (no insulation there).

Considering the Trane XL20i heat pump 4 ton combo rated at 17.75 SEER or the Trane XL16i heat pump 4 ton combo rated at 17.5 SEER. Difference in cost is negligible with the rebates being offered by Trane and FPL and one contractorr is offering a "free" 5-inch media filter with either choice.

What's the trade-off in efficiency and long term reliability between the 2 compressor unit and the single 2-stage compressor unit, given that compressor start-up is the greatest strain on a compressor over the long haul. I mean, if the 2 compressors go on and off, but the dual stage compressor just changes speed, does that mean the dual stage will last longer? Would it also be more efficient?

The old Rheem does manage to keep the RH inside pretty low... measured at 40% earlier today... I wonder if the Trane would do the same. Outside RH runs between 75 and 85% most days.