quality relavent one way links are much better.. make sure they are targetted to your website and not from bad sites. if u link to bad sites, google will put a black mark on your site.
All other things being equal a one way link is best. However, don't hesitate to trade links with a site that is related to you and will put your link in the text on a page that gets traffic. Reciprocal links from sites with several link pages won't help you much.
One way link is better but normally it's not free, and if it's free, the PR shall be low. Exchange links with others will get good PR.
One way link is forever thing for SEO, It will be better always but main thing is how can you get it.