Though I've never skied in SA, I'm planning a trip to Argentina in august, so I've done quite a bit of research on the topic.
Chile typically has better snow, because the resorts are higher. That means drier snow, but also mens you won't be seeing any trees, as everything is above the treeline. It is a bit more expensive than Argentina, specially taking the currency into account.
Argentina seems to have 3 decent options.
Bariloche, as mentioned above, is the biggest. It is also the most crowded and filled with drunken college kids. If you are looking for a place with good nightlife - or, better yet, are a drunker college kid - this should be a pretty good choice.
Las Lenas is mostly comprised of red/black runs. From what I gather, from a pure snowboarding perspective it's the best in Argentina.
Another choice is Chapelco (located near San Martin de Los Andes). Should be much less crowded, have a more diverse mix of green/blue/red and modern lift systems. The city the mountais is located at also seems to pretty lively, with a decent amount of good bars/restaurants.
Myself, I'm hitting Chapelco. My girl has never snowboarded before, so the gentler terrain and good after-ski scenario make it ideal.
Whatever you do, just avoid the last 2 weeks of july. School vacations in Argentina and Brazil make every single resort in south america very crowded.