My instinct is to recommend marketing, but as a marketing strategist, I'd first spend the time making sure that your company can handle the growth or where its capacity limits are and whether you're wasting too much time and energy without a system to automate your company's various operations. Then I'd help you build the strategy to deploy the new revenue most effectively to build your company. Business is multi-faceted, and you have to be able to run on all cylinders to grow. So marketing is extremely important, but it's only one of several areas where a business must be strategic. Always do the comprehensive review to determine your next step (Guess what? I can help if you like).