Id use 5k tops on PPC to give an initial push, and the rest on SEO. For 15k you can make a long term impact on rankings that will keep the traffic pouring in long after the budget is utilized. 15k is a decent budget to work with, heck it cost me less than 2% of that to hit top 5 for "SEO Company" and top 15 for "SEO" and a bunch of others on a new domain.