XS
The two are simply not comparable. AF business class is a different world to premium economy on BA, though PE is better than normal economy the business class option will offer you far, far more comfort. Flights from Europe to South America are difficult, 12-14 hours on a plane can be extremely uncomfortable and from experience I can tell you that a genuine long haul business class product is big help.
The biggest advantage is the lie flat bed, this helps you sleep a lot better than the 'pseudo' economy class seats in PE which just recline a bit. Generally speaking the business class cabin also has a more relaxed atmosphere with more space to move around in.
At the end of the day it boils down to the question of whether you're willing to spend the extra money, I certainly would and personally speaking if I'm paying out of my own money I'll pay for C class on anything which is longhaul.
Good luck