Having worked frequently in McKinsey France, I have also heard that BCG historically was the stronger performing consultancy compared to McKinsey until 4-5 years ago. My undrestanding is that they are now comparable where McKinsey Paris is performing very well and the office has also cotinued to grow. The nuances will be that either firm will be stronger in particular sectors/CAC40 clients/etc. but that is true in any location and easily changes over time.
For the second part of your question. The correlation was somewhat true 5+ years ago where although McKinsey opened Casablanca in the early 2000s the local office was much smaller and Francaphone Africa was still being served by consultants from the Paris office. Today, McKinsey Casablanca, in terms of consultant size, is double the size of BCG Casablanca (i.e., 80 vs. 40 consultants) and has also built a digital hub there for the region.