If you have to pick one, go with the carbon. I do keep 2 bags of Purigen for each filter but also use one at a time (one is regenerated while the other is in the filter). If it costs $20 in your area, I would probably skip it.
Thank for your answers, I will try to find someone who travel between my area and USA (is more common than expected) to see if he can buy me some Purigen there. For now I will use activated carbon as always.