I think more context would help with this question. SEO is not really about one thing being better than another, because it depends on the market, the industry, and your competitors.
If you are in a not very competitive industry, where you are aiming at building content, then on page optimisation will win out.
If you are in a VERY competitive industry, you will HAVE to work hard on your off page optimisation.
So, in general, it is 50/50, but you cant live life like that!