To be honest I think the notion of Souther Hemisphere v Northern Hemisphere in terms of a clear quality divide is demonstrably redundant now. In reality it's more a case of NZ vs the world if you go by the quantum of results. Recently, we've seen Ireland beat both Australia and South Africa, England dominate Australia, whilst South Africa dominate England and Australia dominates Wales, who in turn have been able to beat South Africa. Meanwhile, France continue to be France and produce both brilliance and ineptitude in equal measure.
So if we're to judge the respective club competitions on the basis of the performance of national teams, the only constant is NZ's dominance, so North v South isn't really how we should phrase the question.
This is reflected in their performance in Super Rugby of course, with teams from New Zealand clearly dominating the competition over the course of its history. However, on that same basis, you'd also have to say that whilst the Kiwi club teams would likely be more successful in an integrated international club competition overall, they wouldn't be invincible either - the success of the Tahs, Reds, Bulls and Sharks in Super Rugby over the past 8 years prove that.
