Be careful here since it gets complicated between energy efficiency differences in heating vs. cooling, but also in greenhouse impact of the units themselves. For example, R410A used in modern A/C units is over 2000x more potent as a greenhouse gas than straight CO2, and A/C units leak quite frequently as they age. You also need to factor the greenhouse impact of the energy source of the local power grid during the season when heating/cooling are needed. It's hard to make a solid claim either way without looking at quite a bit of data.
