I hate to say this.. But both! When getting the ball rolling on a start-up, you want to attack all possible marketing outlets. Now, if budget is the bottleneck then I would consider running some analysis on your target audience and see if your users for your service will most likely be using Google to find you (SEO) or will they be more commonly found on Facebook (Facebook ads). You can open a Facebook page for your business and watch it's activity and traffic (analytics).. Take that analysis and then compare it to what your website sees through Google Analytics. This should tell you early on how people are finding your services, in return you will know where to put your money.