It is my point of view, on 2011 I bought an expensive PR package on a reputable PR site and the traffic comming from the PR's was very low and also the value of these links was low too, so I recommend again put this money and effort in a social channels campain it will be more valuable in terms of traffic (targeted traffic) and also link value, authority etc.
PR sites has been absolutelly deprecated by Google and also the traffic comming from them is always less than espected.
This is my personal point of view
PS.: In the 90's we were buying PR packages and our rank was increased in a few weeks, today the rank is not affected by PR sites.
Hope it helps