Hey Tom - in my experience of doing both, South America is a tougher, gruelling and more expensive place to travel in comparison to Southeast Asia.
The continent is bigger, the journeys are longer, and the feeling isn’t as welcoming (although not negative) unlike in Southeast Asia where the hospitality is some of the best in the world.
Costs wise, depending how you do it. South America can be done on a budget from £800 a month but honestly I’d say go for £1200+. South America excursions are more costly, and there are more world class places to visit (Galapagos, Machu Picchu etc)
This group actually isn’t for only backpackers, it’s for all type of travellers - see the about description - so it really depends how you go about it.