I believe it depends on the asset you're promoting. If it's a unique content piece that people (and the press) are going to jump at, then you don't need to throw as much money at it. (However that may require a greater spend to make your asset so epic in the first place).
My gut says: budget for a 50-50 split. You can then incentivise your content creators (perhaps even financially from the promo-pot) to make it more awesome/shareable/newsworthy, without exceeding their creation budget. Net win for everyone. Even the PR folks will be happy because they will have much easier sell-ins to the media/socially, reducing their churn.