I'm ex-Capetonian

The difference between Gauteng winter and Cpt is that Cpt has rain, wind and freezing wind from mountains if ice persists hahaha.. Gauteng winters are dry but sunny so you typically have a decent temperature day from 9am to 4-5pm in the sun. Be prepared for getting use to 0 to -ve temp at night and perhaps consider evaluating rental design(northeast facing) and airflow too e.g. if you need to seal windows and doors yourself (I did this.. makes a few degrees difference and noticeable).
Pretoria is warmer than Johannesburg, and its like a 0.x degrees per km north.. Jhb city is cold because of the lack of vegetation so if u wanna see how cold it is.. go to the neighbourhood goods market in winter and prepare to die/freeze. Centurion is ok.. reason I mentioned EcoPark in particular is that it's one of the coldest places in Centurion i've been told by people and experienced a few times (I work that side).
@Charlie21 EcoPark side to Centurion mall only has delay in peak by the N1 turn off traffic etc. It's not too bad once u know how to drive alternative routes. Most of the traffic is heading towards Centurion West, South (Jhb) and North (PTA) which includes some of the north eastern routes. So yes.. i think for least traffic the area along with CBD side is good but does come with some cost