So first and foremost, take a look at your business & how youre progressing with it. With that youll be able to set a budget for your marketing. Ideally, its best to allocate some money for both SEO & SEM. While SEM is expensive, you can expect quick growth via the same. SEO is relatively cheaper but takes longer to yield effect. Hence, for example, you start with 70:30 split between SEM:SEO and gradually shift to 30:70 over the course of 6 months. Basically, SEM pulls you up and keeps you there for a while until you are able to build genuine ranking via SEO. Once SEO comes into full effect, you can then release funds from SEM & focus on SEO.
I hope ive been able to help.