Make the decision based on the complete system
HVAC capacity should match the building load closely enough to provide comfort under design conditions while allowing stable operating cycles.
Rules of thumb based only on square footage ignore insulation, windows, orientation, air leakage, occupancy and other load factors. The old equipment may also have been oversized.
A load calculation such as ACCA Manual J provides a structured method for estimating room and whole-home heating and cooling loads; equipment selection then has to account for actual manufacturer performance.