The only restriction AFAIK is YPN is currently available to the people of the U.S only. It seems YPN pay more per click than adsense but YPN is not returning ads that are as relevant as the ones that Adsense users are finding. I think in the long run it won't matter if YPN pays more if they can't improve their ad relevancy because they won't get the number of clicks from visitors to see a real difference in their earnings. So more or less the earnings will be same.