In my book main is google by far.
My second best is bing.com , in my experience this traffic converts much better than google traffic, but not much visitors when comparing to google, so google is #1.
Your effort will payoff if you optimize for google, just don't try to trick the google.
Follow their main rules for optimization(onpage) and build just quality and your content related backlinks. Maybe it is harder to find and get such backlinks, but this is what you need to do in order to see google page 1.
One quality and content related link is xxx time stronger then some low quality backlinks, like profile, gusetbook, social bookmark backlinks or so, this not mean that you should not build this kind of backlinks, because you should and you must.