I found that the majority of the Polish HF-bankers are SGH-graduates.
It's a matter of prestige. SGH has tradition and it is public, whereas Kozminski is much more focused on something practical.
Kozminski is better in terms of actual programme, but SGH has the name and traditions. However, in your case, it's unlikely that SGH actually offers anything worthwhile, whereas that course in Kozminski is one of the few good English courses out there.