yahoo gives more importance to on page factors of the site. By the term "more" i mean more than that of google.
I think that just the fact that results on Yahoo are very different than those of Google would tell us there must be more than just links that factor into the results. I'm pretty sure Yahoo pays a lot of attention to domain name keywords, and meta tags and content probably don't hurt.
Yahoo gives more importance to On page factors. One of my sites is ranking for the keyword just by including it in the homepage text.
Every major search engine considers the both things - Backlinks and Page-code. If your page code is not good, then you can't read easily by SEs spiders and if you are not having good link popularity, you can get good hits from search engines.
I'd say, off page optimisation is more important, as long as your know how you are to go about getting indexed, i.e. back link from a highly regarded site in yahoos search engine or having your site featured in yahoos answers in the resource list.