BOTH will always be the safest answer to this famous question...
But if I were to choose from the two I would go with "BACKLINKS"...
Backlinks have really great impact on both
PR and SERP....
If your goal is to rank on SERP, having quality related incoming links would be the major requirement to achieve it....
Even sites with not much content can have a decent ranking on SERP through building quality related backlinks...
That's just me cause we have our own opinions... Cheers!